Diagnostic Aids

Perform this test when directed by QUICK TEST or if directed by other test procedures. This test is used to diagnose.

• HO2S Heater
• Engine Vacuum Systems
• Natural Gas (NG) Module
• Fuel Injector
• Wiring Harness Connector & Circuits (HO2S HTR, INJ 1-8, VPWR & SIG RTN)
• Powertrain Control Module (PCM)

1. 30) DTCs P0135, P0141, P0155 Or P0161: Check HO2S HTR Is Open, Or Shorted To Ground Or VPWR
   
   These DTCs indicate a short to VPWR, or an open VPWR or GND circuit in HO2S heater circuit causing current draw to exceed calibrated limit. HO2S DTC identification is as follows:
   • DTC P0135 Is For HO2S-11
   • DTC P0141 Is For HO2S-12
   • DTC P0155 Is For HO2S-21
   • DTC P0161 Is For HO2S-22

   Possible causes are:

   • Signal Shorted In Wiring Harness Or HO2S
   • Water In Connectors
   • Cut Or Pulled Wires
   • Low Battery Voltage
   • Open In GND Or VPWR Circuit

   Ensure battery is fully charged. Inspect HO2S connectors for loose, damaged or corroded terminals. Repair as necessary. If HO2S connectors are okay, go to next step.

   NOTE: HO2S may be displayed on scan tool as O2S.
2. 31) Perform KOEO Self-Test
   
   Start engine and operate at 2000 RPM for 5 minutes. Turn ignition switch to OFF position. Perform KOEO ON-DEMAND SELF-TEST . If DTC P0135, P0141, P0155 or P0161 are present, go to next step. If specified DTCs are not present, fault may be intermittent. Go to TEST Z .
3. 32) Check For Voltage At HO2S Heater Wiring Harness Connector
   
   Turn ignition switch to OFF position. Disconnect suspect HO2S connector. Turn ignition switch to ON position. Measure voltage between SIG RTN and VPWR terminals at HO2S harness connector. See Fig 1. If voltage is 10.5 volts or less, go to next step. If voltage is more than 10.5 volts, go to step  34).
   Fig 1: Identifying HO2S Connector Terminals

   Courtesy of FORD MOTOR CO.
4. 33) Check For Open Circuit
   
   Turn ignition switch to OFF position. Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Measure resistance between VPWR terminal at HO2S harness connector and PCM connector terminal No. 71. If resistance is less than 4 ohms, go to next step. If resistance is 4 ohms or more, check circuit fuse. If fuse is okay, repair open VPWR circuit.
5. 34) Check HO2S Heater Circuit
   
   Turn ignition switch to OFF position. Measure resistance between HO2S HEATER GND terminal and VPWR terminal at HO2S. If resistance is 3-30 ohms, go to next step. If resistance is not 3-30 ohms, replace HO2S sensor.
6. 35) Check HO2S Case For Short To HTR GND Or VPWR
   
   Measure resistance between HO2S HEATER GND terminal at HO2S and HO2S case. Measure resistance between VPWR terminal at HO2S and HO2S case. If both resistance readings are more than 10 k/ohms, go to next step. If any resistance is 10 k/ohms or less, replace HO2S.
7. 36) Check For Shorts To Other Grounds Or VPWRs In HO2S HTR GND Circuits
   
   Disconnect scan tool from DLC. Ensure PCM is disconnected. Using a DVOM, measure resistance between suspect HO2S harness connector HO2S HTR, PWR GND, SIG RTN and VPWR terminals. See Fig 1. If all resistance measurements are more than 10 k/ohms, go to next step. If any resistance measurement is 10 k/ohms or less, repair short between affected circuits.
8. 37) Check For Open Circuit
   
   Measure resistance between HO2S HEATER GND terminal at suspect HO2S harness connector and appropriate HO2S HTR terminal at PCM connector as follows:
   • Right Front HO2S; Terminal No. 93.
   • Left Front HO2S; Terminal No. 94.
   • Right Rear HO2S; Terminal No. 95.
   • Left Rear HO2S; Terminal No. 96.

   If resistance is 4 ohms or more, repair open circuit or excessive resistance in HO2S wiring harness. If resistance is less than 4 ohms, wiring harness is okay. replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE .

   NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 37) to step 40). No test procedures have been omitted.
9. 40) DTCs P1130, P1131, P1132, P1150, P1151 Or P1152: Upstream HO2S Not Switching
   
   DTCs P1130 and P1150 indicate when an HO2S fails to switch due to a circuit or fuel exceeding a calibrated limit. DTCs P1131, P1132, P1151 and P1152 indicate a problem when fuel control system does not detect HEGO switching for a calibrated time. DTCs P1130, P1131 and P1132 refer to HO2S-11. DTCs P1150, P1151 and P1152 refer to HO2S-21. Possible causes for these faults are:

   • Fuel System
     
     Excessive fuel pressure, leaking or contaminated fuel injectors, leaking fuel pressure regulator, low fuel pressure or engine running out of fuel or vapory recovery system fault.
   • Induction System
     
     Air leaks after MAF sensor, vacuum leaks, PCV system or improperly seated engine oil dipstick.
   • EGR System
     
     Leaking gasket, stuck EGR valve or leaking diaphragm or EVR.
   • Engine Mechanical
     
     Engine oil overfilled, cam timing, cylinder compression or exhaust leaks before or near HO2S.

   Inspect engine for obvious defects in specified systems. Repair as necessary. If no faults are found, go to TEST DC, step  25).

10. 41) Continuous Memory DTC P0171, P0172, P0174 Or P0175: Fuel System At Corrected Fuel Trim
    
    DTCs P0171, P0172, P0174 and P0175 indicate a problem when adaptive fuel strategy senses a lean or rich condition beyond the minimum or maximum calibrated limit. DTCs P0171 (lean) and P0172 (rich) refer to bank No. 1. DTCs P0174 (lean) and P0175 (rich) refer to bank No. 2. Possible causes for these faults are:

    • Fuel System
      
      Excessive fuel pressure, leaking or contaminated fuel injectors, leaking fuel pressure regulator, low fuel pressure or engine running out of fuel or vapory recovery system fault.
    • Induction System
      
      Air leaks after MAF sensor, air intake system obstructions, vacuum leaks, PCV system or improperly seated engine oil dipstick.
    • EGR System
      
      Leaking gasket, stuck EGR valve or leaking diaphragm or EVR.
    • Engine Mechanical
      
      Engine oil overfilled, cam timing, cylinder compression or exhaust leaks before or near HO2S.

    Inspect engine for obvious defects in specified systems. Repair as necessary. If no faults are found, go to TEST DC, step  25).

11. 42) Perform KOER Self-Test
    
    Perform KOER on-demand self-test. See KOER ON-DEMAND SELF-TEST  under QUICK TEST. If KOER DTC P1131, P1132, P1151 or P1152 is present, go to next step. If none of these DTCs are present, proceed as follows:
    • If diagnosing Continuous Memory DTC P1130, P1150, P0171, P0172, P0174 or P0175, go to next step.
    • If diagnosing any other DTCs, fault may be intermittent. Go to TEST Z .
NOTE: Not all CNG vehicles are equipped with Fuel Rail Pressure (FRP) sensor. On vehicles without FRP sensor, use values supplied by fuel rail pressure gauge.
12. 43) Check Fuel Pressure
    Turn ignition switch to OFF position. Connect fuel pressure gauge to Schrader valve located on fuel rail. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Observe fuel pressure gauge. Turn ignition switch to ON position and note fuel pressure reading. Start engine and allow to idle. Note fuel pressure reading. Increase engine speed to 2500 RPM for one minute. Note fuel pressure reading. If fuel pressure readings are 80-120 psi (552-827 kPa), go to next step. If fuel pressure readings are not 80-120 psi (552-827 kPa), go to TEST HB .
13. 44) Check Fuel System For Pressure Stability
    
    With fuel pressure gauge installed, cycle ignition switch from OFF to ON position 2 times to pressurize fuel system (DO NOT start engine). If fuel pressure does not remain within 10 psi (69 kPa) of pressure reading for at least 2 minutes, go to TEST HB . If fuel pressure remains within 10 psi (69 kPa) of highest fuel pressure reading for 2 minutes, proceed as follows:
    • For no-start vehicles, go to next step.
    • For DTCs P1130, P1150, P0171 P0172, P0174 and P0175, go to next step.
    • For all other DTCs, go to step  62).
14. 45) Check Fuel System For Pressure Stability With Ignition Switch Turned To ON Position
    
    Using scan tool, access FRP PID (if available) from PID/DATA MONITOR & RECORD menu. If vehicle is not equipped with FRP sensor, install Natural Gas Fuel Rail Pressure Gauge Tool Kit (134-00114) to fuel rail Schrader valve. Cycle ignition from OFF to ON (DO NOT start engine). Monitor fuel pressure reading. If fuel pressure does not remain within 10 psi (69 kPa) of highest fuel pressure measurement for at least 10 seconds, go to step  56). If fuel pressure remains within 10 psi of original pressure for at least 10 seconds, proceed as follows:
    • For no-start vehicles, go to next step.
    • For DTCs P1130, P1150, P0171 and P0174, go to step  55).
15. 46) Check Ability Of Injectors To Deliver Fuel
    
    Using scan tool, access FRP PID (if available) from PID/DATA MONITOR & RECORD menu. If vehicle is not equipped with FRP sensor, install Natural Gas Fuel Rail Pressure Gauge Tool Kit (134-00114) to fuel rail Schrader valve. Cycle ignition switch from OFF to ON position 2 times to pressurize fuel system (DO NOT start engine). Record fuel pressure reading. Locate and disconnect Inertia Fuel Shutoff (IFS) switch. Crank engine for 10 seconds. If fuel pressure reading remains within 20 psi (137 kPa) of original pressure reading, remove fuel pressure gauge and reconnect IFS switch. Go to next step. If fuel pressure drops more than 20 psi (137 kPa) from original pressure reading, EEC-V system is not the cause of no-start. Diagnose no-start symptom. See NO START/NORMAL CRANK under SYMPTOMS in TROUBLE SHOOTING - NO CODES - EEC-V (GASOLINE) article.
16. 47) Check For VREF At NG Module
    
    Turn ignition switch to OFF position. Disconnect PCM connector(s). Locate NG module 60-pin connector. See NATURAL GAS VEHICLE MODULE LOCATION  table. Turn ignition switch to ON position. Using a DVOM, measure voltage between NG module harness connector VREF and SIG RTN terminals. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 2. If voltage is 4-6 volts, go to next step if engine is a no-start, or step  57) for all others. If voltage is not 4-6 volts, go to step  50).
    NATURAL GAS VEHICLE MODULE LOCATION
    
    

    Application	Location
    Crown Victoria	Front Of Radiator Support
    Econoline	In Left Side Of Engine Compartment, On Inner Fender
    Pickup F150 & F250	In Front Of Engine

    NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION
    
    

    Application		NG Module Terminal No.
    PWR GND		40 & 60
    SIG RTN
    Fuel Injector No. 1		58
    Fuel Injector No. 2		59
    Fuel Injector No. 3		39
    Fuel Injector No. 4		35
    Fuel Injector No. 5		33
    Fuel Injector No. 6		42
    Fuel Injector No. 7		53
    Fuel Injector No. 8		54
    VPWR		37 & 57
    VREF		26

    Fig 2: Identifying NG Module Circuits & Connector Terminals

    Courtesy of FORD MOTOR CO.
17. 48) Check For Open PWR GND Circuit
    
    Turn ignition switch to OFF position. Disconnect NG module 60-pin connector. Using a DVOM, measure resistance of PWR GND circuit between NG module harness connector terminal and negative battery terminal. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 2. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open in PWR GND circuit.
18. 49) Check For Open In PWR GND Circuit In NG Module
    
    With NG module disconnected, measure resistance between NG module PWR GND and SIG RTN terminals. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 2. If resistance is 5 ohms or more, replace NG module. If resistance is less than 5 ohms and engine is a no-start, go to step  57). On all others, reconnect PCM and NG module and go to TEST A, step  6).
19. 50) Check VREF Voltage
    
    If voltage in step  47) was less than 6 volts or less, go to next step. If voltage was more than 6 volts, go to step  55).
20. 51) Check Battery Voltage
    
    Using a DVOM, measure voltage between battery terminals. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, check charging system and battery. See appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS. Repair as necessary.
21. 52) Check VPWR Circuit At NG Module
    
    Turn ignition switch to ON position. Using a DVOM, measure voltage between NG module connector PWR GND and VPWR terminals. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 2. If voltage is 10.5 volts or less, go to next step. If voltage is more than 10.5 volts, go to step  54).
22. 53) Check Voltage Between VPWR At NG Module & Battery Ground
    
    Using a DVOM, measure voltage between negative battery terminal and NG module connector VPWR terminal. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 2. If voltage is 10.5 volts or less, repair open in VPWR circuit. If voltage is more than 10.5 volts, repair open in PWR GND circuit.
23. 54) Check VREF Voltage To Fuel Tank Pressure Sensor
    
    Turn ignition switch to OFF position. Locate and disconnect Fuel Tank Pressure (FTP) sensor connector. See FUEL TANK PRESSURE SENSOR LOCATION  table. Turn ignition switch to ON position. Using a DVOM, measure voltage between FTP sensor harness connector SIG RTN and VREF terminals. See Fig 3. If voltage is 4-6 volts, replace FTP sensor. If voltage is not 4-6 volts, go to step  56).
    FUEL TANK PRESSURE SENSOR LOCATION
    
    

    Application	Location
    Crown Victoria	On Upper Fuel Tank Assembly
    Econoline	Under Left Center Of Vehicle
    Pickup	Part Of Fuel Pump Assembly

    Fig 3: Identifying FTP Connector Terminals

    Courtesy of FORD MOTOR CO.
24. 55) Check VREF Circuit For Short To VPWR In Harness
    
    Ensure NG module and FTP sensor are disconnected. Turn ignition switch to ON position. Using a DVOM, measure voltage between NG module harness connector PWR GND and VREF terminals. See Fig 3. If voltage is less than one volt, replace NG module. If voltage is one volt or more, repair short to VPWR in VREF circuit.
25. 56) Check VREF Circuit For Short To PWR GND
    
    Turn ignition switch to OFF position. Disconnect NG module connector. Using a DVOM, measure resistance between NG module connector CASE GND, PWR GND, SIG RTN and VREF terminals. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION . See Fig 3. If all resistance measurements are more than 10 k/ohms, replace NG module. If any resistance measurement is 10 k/ohms or less, repair short to ground in VREF circuit.
26. 57) Check Resistance Of Injectors & Related Circuits
    
    Ensure fuel injector harness connectors are connected. Leave ignition switch turned to OFF position and NG module disconnected. Using a DVOM, measure resistance between NG module harness connector VPWR terminal and suspect fuel injector terminal. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION  table. See Fig 3. If resistance is 3-6 ohms, go to step  60). If resistance is not 3-6 ohms, go to next step.
27. 58) Check Fuel Injector Harness Resistance Between NG Module & Fuel Injector
    
    Disconnect suspect fuel injector harness connector. Using a DVOM, measure resistance of fuel injector SIG RTN and VPWR circuits between suspect fuel injector harness connector terminal and appropriate NG module harness connector terminal. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION  table. See Fig 2. If both resistance measurements are less than 5 ohms, go to next step. If any resistance measurement is 5 ohms or more, repair open in affected SIG RTN or VPWR circuit.
28. 59) Check Fuel Injector SIG RTN Circuit For Short To VPWR Or Ground
    
    Disconnect scan tool from DLC. Ensure suspect fuel injector harness connector is disconnected. Using a DVOM, measure resistance between NG module harness connector suspect fuel injector SIG RTN terminal and PWR GND and VPWR terminals. See NG MODULE FUEL INJECTOR TERMINAL IDENTIFICATION  table. See Fig 2. If any resistance measurement is 10 k/ohms or less, repair short between circuits. If all resistance measurements are more than 10 k/ohms, go to step  61) if DTC P0172 or P0175 is present, or replace faulty fuel injector(s) if DTC P0172 or P0175 is not present.
29. 60) Check Injector Driver Signal At NG Module
    
    Ensure ignition switch is turned to OFF position. Connect NG module harness connector. Connect 12-volt test light between suspect fuel injector harness connector SIG RTN and VPWR terminals. While cranking engine, observe test light. If test light does not glow dimly (no light or continuous bright light) while cranking or running engine, go to next step. If test light glows dimly, PCM and NG module circuits are okay. If engine runs rough, see ROUGH OR ROLLING IDLE, STALLS, MISSES, HESITATION under SYMPTOMS in TROUBLE SHOOTING - NO CODES - EEC-V (GASOLINE) article. If engine is a no-start, go to TEST A, step  4). For all others, go to step  64).
30. 61) Check PCM Fuel Injector Harness Circuit Resistance
    
    Ensure ignition switch is turned to OFF position. Ensure NG module and PCM harness connectors are disconnected. Inspect connectors for loose, damaged or corroded terminals. Repair as necessary. Using a DVOM, measure resistance of VPWR circuit between PCM harness connector terminals and NG module harness connector terminals. See PCM TO NG MODULE TERMINALS  table. See Fig 2. Using a DVOM, measure resistance of fuel injector circuits between PCM harness connector terminal and NG module harness connector terminal. If all resistance measurements are less than 5 ohms, go to next step. If any resistance measurement is 5 ohms or more, repair open in affected circuit.
    PCM TO NG MODULE TERMINALS
    
    

    Cylinder No.	PCM Terminal No.	NG Module Terminal No.
    1	75	3
    2	101	4
    3	74	5
    4	100	23
    5	73	24
    6	99	25
    7	72	43
    8	98	44

31. 62) Check PCM Fuel Injector Circuit For Short To PWR Or GND
    
    Measure resistance between NG module connector fuel injector terminals (No. 3, 4, 5, 23, 24, 25, 43 and/or 44) and terminals No. 37, 40, 57 and 60. See PCM TO NG MODULE TERMINALS  table. Measure resistance between PCM connector fuel injector terminals (No. 75, 101, 74, 100, 73, 99, 72 and/or 98) and terminals No. 71 and 103. See PCM TO NG MODULE TERMINALS table. If all resistance readings are more than 10 k/ohms and DTCs P0172 and P0175 are present, go to step  64). If all resistance readings are more than 10 k/ohms and DTCs P0172 and P0175 are not present, go to next step. If any resistance reading is 10 k/ohms or less, repair short circuit.
32. 63) Check Fuel Injector Driver Signal From PCM
    
    Ensure ignition switch is turned to OFF position. Connect PCM harness connector. Ensure NG module is disconnected. Connect 12-volt test light between NG module connector terminal No. 57 (VPWR) and terminals No. 3, 4, 5, 23, 24, 25, 43 and 44. See Fig 2. While cranking engine, observe test light. If test light does not glow dimly or flicker, replace PCM. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE . If test light glows dimly or flickers, engine runs okay or you were directed here for DTCs, go to next step. If test light glows dimly or flickers and engine is a no-start, replace NG module. If test light glows dimly or flickers and engine runs rough, diagnose by symptom. See ROUGH OR ROLLING IDLE, STALLS, MISSES, HESITATION under SYMPTOMS in TROUBLE SHOOTING - NO CODES - EEC-V (GASOLINE) article.
33. 64) Check Fuel Injector Flow & Leakage
    
    Turn ignition switch to OFF position. Reconnect all connectors. Use Rotunda injector tester from Fuel Tester Kit (113-00114) to flow test fuel injectors. Follow injector tester instructions. If fuel injector flow or leakage rate is not okay, replace defective fuel injector(s). If flow rate for each fuel injector is okay, proceed as follows:
    • For DTCs P0171, P0172, P0174 and P0175, fault is intermittent. Go to TEST Z .
    • For DTCs P1130 and P1150, go to step  66).
    • For all other DTCs, go to next step.
34. 65) Check Cylinder Compression
    
    Using compression gauge, check cylinder compression. If cylinder compression is not okay, repair engine as necessary. If compression is okay, go to next step (DTCs P1130, P1131, P1150 and P1151) or step  71) (DTCs P1132 and P1152).
35. 66) Check HO2S Integrity
    
    DTCs P1130, P1150, P1131, and P1151 indicate HO2S switches slow or doesn't switch, is always lean or fuel is at adaptive limit. Possible causes are:
    • Moisture Inside HO2S Causing Short To Ground
    • HO2S Coated With Contaminates
    • HO2S Circuit Open Or Shorted To Ground

    Turn ignition switch to OFF position. Inspect HO2S and wiring harness for damage and contamination from oil, water, etc. Repair as necessary. If no visible problems are found, start engine and operate at 2000 RPM for 2 minutes. Turn ignition switch to OFF position. Using scan tool, Perform KOER on-demand self-test. See KOER ON-DEMAND SELF-TEST  under QUICK TEST. If DTC P1131 or P1151 is present, go to next step. If DTC P1131 or P1151 is not present, HO2S system is functioning properly at this time. Faults cannot be duplicated or identified at this time. Go to TEST Z .

36. 67) Check HO2S Ability To Generate Correct Voltage
    
    Turn ignition switch to OFF position. Disconnect suspect HO2S connector. Using DVOM, measure voltage between HO2S SIG terminal and SIG RTN terminal at HO2S harness connector. See Fig 1. Start engine and operate at 2000 RPM for 2 minutes. Perform KOER on-demand self-test. See KOER ON-DEMAND SELF-TEST  under QUICK TEST while monitoring voltage. If voltage is more than .5 volt during, or at end of, test, go to next step. If voltage stays at .5 volt or less during test, replace HO2S sensor.
37. 68) Check HO2S SIG & SIG RTN Circuits For Open
    
    Turn ignition switch to OFF position. Disconnect PCM connector. Disconnect suspect HO2S harness connector. Inspect both ends of connector for loose, damaged or corroded terminals. Repair as necessary. Using a DVOM, measure resistance of HO2S SIG and SIG RTN circuits between appropriate PCM harness connector terminal and suspect HO2S harness connector. See CNG HO2S PCM TERMINAL IDENTIFICATION . See Figure and Fig 1 . If both resistance measurements are less than 5 ohms, go to next step. If either resistance measurement is 5 ohms or more, repair open in HO2S SIG or SIG RTN circuit.
38. 69) Check HO2S Circuit For Short To Ground
    
    Measure resistance between appropriate HO2S SIG circuit terminal at PCM connector and negative battery terminal. If resistance is more than 10 k/ohms, go to next step. If resistance is 10 k/ohms or less, repair short to ground.
39. 70) Check HO2S For Short To Ground
    
    Measure resistance between appropriate HO2S SIGNAL terminal and terminals No. 91 and 103 at PCM connector. If any resistance reading is 10 k/ohms or less, replace HO2S. If both resistance readings are more than is 10 k/ohms, proceed as follows:
    • For DTCs P1130 and P1150, go to next step.
    • For Continuous Memory DTCs P1131 and P1151, go to step  76).
    • For KOER DTCs P1131 and P1151, replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE .
40. 71) Check For DTCs P1132 & P1152 With P1130 & P1150
    
    Start engine, and warm it to normal operating temperature. With scan tool connected, Perform KOER on-demand self-test. See KOER ON-DEMAND SELF-TEST  under QUICK TEST. If DTC P1132 or P1152 is present, go to next step. If DTC P1132 or P1152 is not present, fault is intermittent. Go to TEST Z .
41. 72) Check For HO2S SIG Circuit For Short To VPWR & VREF
    
    DTCs P1130, P1150 and/or P1132, P1152 indicate HO2S is always rich. DTCs P1130 and P1132 are for right front HO2S. DTCs P1150 and P1152 are for left front HO2S. Possible causes are:
    • Moisture Inside HO2S Wiring Harness Connector
    • HO2S Circuit Shorted To Power
    NOTE: HO2S may be displayed on scan tool as O2S.

    Turn ignition switch to ON position. Using scan tool, select appropriate HO2S PID for DTC generated:

    • PID for DTC P1130 and P1132 is HO2S-11.
    • PID for DTC P1150 and P1152 is HO2S-21.

    If HO2S PID voltage is 1-4 volts, go to next step. If HO2S PID voltage is not 1-4 volts, go to step  75).

42. 73) Check HO2S Circuit For Short To VPWR
    
    Turn ignition switch to OFF position. Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Disconnect suspect HO2S connector. Disconnect scan tool from DLC. For DTCs P1130 and P1132, measure resistance between PCM connector terminals No. 60 and 71. For DTCs P1150 and P1152, measure resistance between PCM connector terminals No. 87 and 71. If both resistance readings are more than 10 k/ohms, reconnect PCM connector and go to next step. If any resistance reading is 10 k/ohms or less, repair short to power.
43. 74) Check HO2S Signal
    
    Ensure suspect HO2S is disconnected. Turn ignition switch to ON position. Using scan tool, select HO2S PID for DTC generated. If PID voltage is less than .2 volt, replace HO2S. If PID voltage is .2 volt or more, replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE .
44. 75) Attempt To Generate DTC P1131 & P1151
    
    Connect a jumper wire between HO2S SIGNAL terminal at HO2S harness connector and negative battery terminal. Perform KOER on-demand self-test. See KOER ON-DEMAND SELF-TEST  under QUICK TEST. If DTC P1131 or P1151 is present, remove jumper wire and go to next step. If DTC P1131 or P1151 is not present, check PCM connector terminals and repair as necessary. If PCM connector is okay, replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE .
45. 76) HO2S Voltage Check
    
    Connect DVOM between HO2S SIGNAL terminal and SIG RTN terminal at HO2S harness connector. Disconnect any vacuum hose from vacuum tree. Start engine and operate at 2000 RPM. If DVOM voltage is less than .4 volt within 30 seconds, reconnect vacuum hose and all connectors, and go to next step. If DVOM voltage is not less than .4 volt, replace HO2S.
NOTE: HO2S PID may be displayed on scan tool as O2S.
46. 77) Monitor HO2S PID
    Start engine and allow to idle. Using scan tool, select HO2S PID for suspect HO2S. Observe PID voltage while wiggling and bending wiring harness between HO2S and PCM. If PID voltage stays high (more than .45 volt) or stays low (less than .45 volt), isolate cause of lack of HO2S switches. Repair as necessary. If PID voltage switches, go to next step.
47. 78) Monitor HO2S PID During Test Drive
    
    Leave scan tool connected to DLC. Using an assistant, test drive vehicle under various conditions while observing HO2S PID voltage. If PID voltage switches from about .4 to .6 volt, system is okay and testing is complete. If voltage does not switch from about .4 to .6 volt, replace HO2S.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 78) to step 90). No test procedures have been omitted.
48. 90) KOER DTC P0127
    
    DTC P1127 indicates that downstream HO2S heaters were not cycled on during KOER self-test due to exhaust temperatures falling below a minimum calibrated value. Possible causes are:
    • Engine Not At Operating Temperature
    • Exhaust System Too Cool

    Connect scan tool to Data Link Connector (DLC). Using scan tool, access all HO2S HTR PIDs from PID/DATA MONITOR & RECORD menu. If all HO2S HTR PIDs switch or indicate ON, perform KOER ON-DEMAND SELF-TEST . If any HO2S HTR PIDs do not switch or indicate OFF, operate engine until all HO2S HTR PIDs switch or indicate ON. Repeat KOER ON-DEMAND SELF-TEST .

    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 90) to step 100). No test procedures have been omitted.
49. 100) KOER P1128 & P1129
    
    These DTCs indicate that during KOER self-test, one or both HO2S signals were crossed from bank to bank. DTC P1128 refers to HO2S-11 or HO2S-21 in front of converter. DTC P1129 refers to HO2S-12 or HO2S-22 behind converter. Possible causes for these faults are:
    • Crossed HO2S Connectors
    • Crossed HO2S Wiring At HO2S Connector
    • Crossed HO2S Wiring At PCM Connector

    Turn ignition switch to OFF position. Disconnect suspect HO2S harness connector. Inspect connector for indication of crossed wires or incorrect installation. Repair as necessary. If no faults are found, go to next step.

50. 101) Verify Proper HO2S SIG Terminal Location
    
    Disconnect PCM connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Disconnect both suspect HO2S harness connectors. HO2S DTC identification is as follows:
    • DTC P1128 is for upstream HO2S-11 and HO2S-21.
    • DTC P1129 is for downstream HO2S-12 and HO2S-22.

    Refer to illustrations for HO2S identification. See Figure and Figure . Using a DVOM, measure resistance of HO2S SIG circuit between suspect HO2S harness connector and appropriate PCM harness connector terminal. See CNG HO2S PCM TERMINAL IDENTIFICATION . See Figure, Figure and Figure . If resistance is less than 5 ohms, fault is intermittent and cannot be duplicated at this time. If resistance is 5 ohms or more, HO2S wiring is crossed. Recheck all HO2S wiring terminal locations at PCM and HO2S connectors. Repair as necessary.