2000 Mazda B2500 SX, AutomaticFACTORY OEM SERVICE PROCEDURE

Adaptive Fuel Control

DTC P0171, P0174 DETECTION CONDITION AND POSSIBLE CAUSE

DTC P0171, P0174 ADAPTIVE FUEL CONTROL
DETECTION CONDITION 
  • P0171 indicates the fuel/air ratio is too lean (RH).
  • P0174 indicates the fuel/air ratio is too lean (LH).
POSSIBLE CAUSE 
  • Fuel system:
    • Low fuel pressure or running out of fuel.
    • Leaking fuel injector(s).
    • Leaking fuel pressure regulator.
    • Vapor recovery system.
    • Contaminated fuel injector(s).
  • Induction system:
    • Air leaks after the MAF.
    • Vacuum leaks (vacuum lines and gaskets).
    • Restricted air inlet.
    • Improperly seated engine oil dipstick.
  • EGR system:
    • Leaking gasket.
    • Stuck EGR valve.
    • Leaking diaphragm or EGR Vacuum Regulator (EGRVR).
  • Base engine:
    • Oil overfill.
    • Cam timing.
    • Cylinder compression.
    • Exhaust leaks before or near the HO2S(s).
HINT  Fuel System Monitor 
  • The fuel control system uses fuel trim tables stored in the PCM to compensate for variability in fuel system components due to normal wear and aging.
  • The fuel trim strategy learns deviations during closed-loop engine operation.
  • The fuel trim has two means of adapting; a long term fuel trim and a short term fuel trim system.
  • Inputs from EATC, IAT and MAF sensors are required to activate the fuel trim system.
  • The HO2S detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.
  • A fuel trim factor is added to the fuel injector pulse width calculation according to the long and short term fuel trims.
  • The long term trim values at their rich or lean limits indicate a fuel system malfunction.
DTC P0171, P0174 INSPECTION

STEP INSPECTION ACTION
1
  • Was PCM Quick Test performed? 
 Yes If Misfire DTC (P0300-P0306) is present or drive ability concern is present, go to step 6.
If any other DTC is present, go to appropriate DTC inspection.
Otherwise, go to next step.
No Perform PCM Quick Test.
2 IDENTIFY TRIGGER DTC FOR FREEZE FRAME DATA
  • Is DTC P0171 or P0174 on FREEZE FRAME DATA? 
 Yes Go to next step.
No Go to troubleshooting procedure for DTCs on FREEZE FRAME DATA.
3 VERIFY CURRENT INPUT SIGNAL STATUS
  • Key ON, Engine Running (KOER).
  • Access ECT V, MAF V, TP V and VSS PIDs using NGS Tester.
  • Is there any signal that is far out of specification during KOER? 
 Yes Inspect suspected sensor and excessive resistance in related wiring harnesses. Repair or replace if necessary.
Go to step 14.
No Go to next step.
4 VERIFY CURRENT INPUT SIGNAL STATUS UNDER TROUBLE CONDITION
  • Access ECT V, MAF V, TP V and VSS PIDs while simulating FREEZE FRAME DATA condition.
  • Is there any input signal which causes drastic changes? 
 Yes Inspect suspected sensor and related wiring harnesses. Repair or replace if necessary.
Go to step 14.
No Go to next step.
5 VERIFY CURRENT INPUT SIGNAL STATUS OF FRONT HO2S (RH, LH)
  • Access O2S11 PID for P0171 or O2S21 PID for P0174.
  • Check PID under following accelerator pedal condition (in PARK):
    • Sudden depress of accelerator pedal (rich condition) should read more than 0.45 V.
    • Sudden release of accelerator pedal (lean condition) should read less than 0.45 V.
  • Is PID reading correctly? 
 Yes Inspect for air suction at following locations due to cracks, damage, and loose parts:
  • From air cleaner to throttle body.
  • From throttle body to dynamic chamber.
  • From dynamic chamber to intake manifold.
  • Vacuum hoses
NOTE: Engine speed may change when rust penetrating agent is sprayed on the air suction area.

Repair or replace any faulty part.
Go to step 14.
No Visually inspect for any gas leakage between exhaust manifold and H20S. If there is no leakage, replace HO2S.
See HEATED OXYGEN SENSOR (HO2S) in REMOVAL, OVERHAUL AND INSTALLATION article in Engine Performance .
Go to step 14.
6 INSPECT MAF V PID
  • Key ON, Engine Running (KOER).
  • Access MAF V PID using NGS Tester.
  • Verify that MAF V PID changes quickly according to race engine RPM.
  • Is MAF V PID response OK? 
 Yes Go to next step.
No Replace MAF sensor.
Go to step 14.
7 INSPECT FOR EXCESSIVE AIR SUCTION OF INTAKE AIR SYSTEM
  • Visually inspect air intake system for cracks, damage, or loose parts.
  • Is there a problem? 
 Yes Repair or replace source of air suction.
Go to step 14.
No Go to next step.
8 INSPECT FUEL LINE PRESSURE
  • Key OFF.
  • Connect fuel pressure gauge.
  • Key ON, Engine Running (KOER).
  • Is fuel pressure within 30-65 psi (207-448 kPa)? 
 Yes Go to step 12.
No Go to next step
9 INSPECT FUEL PUMP CIRCUIT
  • Key OFF.
  • Inspect fuel pump circuit for open or poor connection.
  • Is circuit OK? 
 Yes Go to next step.
No Repair or replace suspected circuit.
Go to step 14.
10 INSPECT FUEL LINE FROM FUEL PUMP TO FUEL DELIVERY LINE
  • Visually inspect fuel line for any leakage.
  • Is any fuel leakage found? 
 Yes Replace fuel line.
Go to step 14.
No Go to next step.
11 INSPECT FUEL FILTER
  • Inspect fuel filter for:
    • Restriction or clogging at high pressure side.
    • Foreign material or stains at low pressure side.
  • Is anything found? 
 Yes If anything is found at high pressure side, replace fuel filter.
See FUEL FILTER (B2500, B3000 & B4000) in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
If anything is found at low pressure side, clean fuel tank and filter.
See FUEL TANK in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
Go to step 14.
No Replace fuel pump.
See FUEL PUMP in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
Go to step 14.
12 INSPECT ENGINE COMPRESSION
  • Perform an Engine Compression test.
  • Is it OK? 
 Yes Go to next step.
No Overhaul engine.
13 INSPECT FUEL INJECTOR OPERATION
  • Remove injector from suspected bank and swap with injector from other bank.
  • Key ON, Engine Running.
  • Is fuel trim system lean for bank which has suspected injector? 
 Yes Key OFF
Replace fuel injector.
See FUEL RAILS & INJECTORS in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
Go to step 14.
No Go to next step.
14 DRIVE CYCLE
  • Make sure to reconnect all disconnected connectors.
  • Clear DTCs from PCM memory using NGS tester generic OBD-II function.
  • Perform Fuel Monitor or HO2S Monitor Drive Cycle.

    See DRIVE CYCLES .

  • Retrieve pending code.
  • Is same DTC present? 
 Yes Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to next step.
No Go to next step.
15 VERIFY TROUBLESHOOTING OF DTCs COMPLETED
  • Perform PCM Quick Test.
  • Is any DTC present? 
 Yes go to applicable DTC inspection.
No Troubleshooting complete.


DTC P0172, P0175 DETECTION CONDITION AND POSSIBLE CAUSE

DTC P0172, P0175 ADAPTIVE FUEL CONTROL
DETECTION CONDITION 
  • P0172 indicates the fuel/air ratio is too rich (RH).
  • P0175 indicates the fuel/air ratio is too rich (LH).
POSSIBLE CAUSE 
  • Fuel system:
    • Excessive fuel pressure.
    • Vapor recovery system.
  • Induction system:
    • Restricted air inlet.
    • PCV system.
    • Improperly seated engine oil dipstick.
  • EGR system:
    • Leaking gasket.
    • Stuck EGR valve.
    • Leaking diaphragm or EGRVR.
  • Base engine:
    • Oil overfill.
    • Cam timing.
    • Cylinder compression.
    • Exhaust leaks before or near the HO2S(s).
HINT  Fuel System Monitor 
  • The fuel control system uses fuel trim tables stored in the PCM to compensate for variability in fuel system components due to normal wear and aging.
  • The fuel trim strategy learns deviations during closed-loop engine operation.
  • The fuel trim has two means of adapting; a long term fuel trim and a short term fuel trim system.
  • Inputs from EATC, IAT and MAF sensors are required to activate the fuel trim system.
  • The HO2S detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.
  • A fuel, trim factor is added to the fuel injector pulse width calculation according to the long and short term fuel trims.
  • The long term trim values at their rich or lean limits indicate a fuel system malfunction.
DTC P0172, P0175 INSPECTION

STEP INSPECTION ACTION
1
  • Was PCM Quick Test performed? 
 Yes If drive ability concern or rough idle is present, go to step 7.
If any other DTC is present, go to appropriate DTC troubleshooting procedure.
Otherwise, go to next step.
No Perform PCM Quick Test.
2 IDENTIFY TRIGGER DTC FOR FREEZE FRAME DATA
  • Key ON, Engine OFF (KOEO).
  • Is DTC P0172 or P0175 on FREEZE FRAME DATA? 
 Yes Go to next step.
No Go to troubleshooting procedure for DTCs on FREEZE FRAME DATA.
3 VERIFY CURRENT INPUT SIGNAL STATUS
  • Key ON, Engine Running (KOER).
  • Access ECT V, MAFV, TP V and VS PIDs as well as BARO and LONGFT using NGS Tester.
  • Is there any signal that is far out of specification during KOER? 
 Yes Inspect suspected sensor and excessive resistance in related wiring harnesses.
Repair or replace if necessary.
Go to step 9.
No Go to next step.
4 VERIFY CURRENT INPUT SIGNAL STATUS UNDER TROUBLE CONDITION
  • Access ECT V, MAF V, TP V and VS PIDs while simulating FREEZE FRAME DATA condition.
  • Is there any input signal which causes drastic changes? 
 Yes Inspect suspected sensor and related wiring harnesses. Repair or replace if necessary.
Go to step 9.
No Go to next step.
5 VERIFY CURRENT INPUT SIGNAL STATUS OF FRONT HO2S (RH, LH)
  • Access O2S11 PID for P0172 or O2S21 PID for P0175.
  • Check PID under following accelerator pedal condition (in PARK):
    • Sudden depress of accelerator pedal (rich condition) should read more than 0.45 V.
    • Sudden release of accelerator pedal (lean condition) should read less than 0.45 V.
  • Is PID reading correctly? 
 Yes Go to next step.
No Replace suspected HO2S.
See HEATED OXYGEN SENSOR (HO2S) in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
Go to step 9.
6 INSPECT FUEL LINE PRESSURE
  • Key OFF, Engine OFF (KOEO).
  • Connect fuel pressure gauge.
  • Key ON, Engine Running.
  • Is fuel pressure within 30-65 psi (207-448 kPa)? 
 Yes Go to next step.
No Replace fuel pump.
See FUEL PUMP in REMOVAL, OVERHAUL, AND INSTALLATION article in Engine Performance .
Go to step 9.
7 INSPECT EVAP CANISTER PURGE VALVE
  • Key OFF, Engine OFF (KOEO).
  • Disconnect both hoses from evap. canister purge valve.
  • Blow air through EVAP canister purge valve.
  • Does air blow through? 
 Yes Replace EVAP canister purge valve.
See ELECTRICAL COMPONENT LOCATOR .
Go to step 9.
No Go to next step.
8 INSPECT PCV VALVE OPERATION
  • Inspect PCV valve for proper operation.
  • Is PCV valve OK? 
 Yes Go to next step.
No Replace PCV valve.
Go to next step.
9 DRIVE CYCLE
  • Make sure to reconnect all disconnected connectors.
  • Clear DTCs from PCM memory using NGS tester generic OBD-II function.
  • Perform Fuel Monitor or HO2S Monitor Drive Cycle. DRIVE CYCLES .
  • Retrieve pending code.
  • Is same DTC present? 
 Yes Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to next step.
No Go to next step.
10 VERIFY TROUBLESHOOTING OF DTCs COMPLETED
  • Perform PCM Quick Test.
  • Is any DTC present? 
 Yes Go to applicable DTC inspection.
No Troubleshooting complete.
Factory Service Tools

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.

Shop Manual-Ready ToolsCompare Repair QuoteQuote Shield Pro

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

Rather have a pro handle this?

A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.

Comes to youSame-day available12-month warranty
Get a Free Quote

No obligation. See pricing before you book.