Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 79 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.4L)INSPECTION/MAINTENANCE PROCEDURESINSPECTION/MAINTENANCE SYSTEM DTCSDESCRIPTION
1999 GMC Forward Control 5.7 R, Automatic
Inspection/Maintenance System DTCS: Description
1999 GMC Forward Control 5.7 R, AutomaticSECTION Description
WARNING: This page is about a different car, the 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, 2002 Chevrolet Monte Carlo, and 2002 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
The following DTCs are required to set system status to YES:
Catalyst System
- P0420 - Catalyst System Low Efficiency
EGR System
- P0401- EGR Flow Insufficient
- P0404 - EGR Open Position Performance
- P0405 - EGR Position Sensor Circuit - Low Voltage
- P1404 - EGR Closed Position Performance
EVAP System
- P0440 - EVAP System
- P0442 - EVAP System Small Leak Detected
- P0446 - EVAP Vent System Performance
- P0447 - EVAP Vent Valve Control Circuit - Low Voltage
- P1441 - EVAP System Flow During Non-Purge
Oxygen Sensor System
- P0133 - HO2S/O2S Slow Response (Sensor 1)
- P0140 - HO2S Circuit Insufficient Activity (Sensor 2)
- P1133 - HO2S Insufficient Switching (Sensor 1)
- P1134 - HO2S Transition Time Ratio (Sensor 1)
Oxygen Sensor Heater System
- P0135 - HO2S Heater Performance (Sensor 1)
- P0141 - HO2S Heater Performance (Sensor 2)
Diagnose affected DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
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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.