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 DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P0100-P0134DTC P0125DTC P0125: ECT EXCESSIVE TIME TO ENTER CLOSED LOOP FUEL CONTROL (3.8L "C", "F", "G", "H" & "W" BODIES)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedures
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
- Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
- Allow engine to cool completely. Turn engine on and allow it to idle. Using scan tool, monitor ENG COOL TEMP. If temperature increases to greater than 70°F (21°C) within 2 minutes, see DIAGNOSTIC AIDS . If specified temperature is not reached within 2 minutes, go to next step.
- If any ECT related DTCs are set, diagnose affected DTC before proceeding. If no ECT related DTCs are set, go to next step.
- Allow engine to cool. Turn ignition on and observe cooling fans. If cooling fans are off, go to next step. If cooling fans are on, diagnose and repair cooling fan problem.
- Check coolant level. If coolant level is low, go to step 9. If coolant level is okay, go to next step.
- Check thermostat operation. If thermostat is operating correctly, go to next step. If thermostat is not operating correctly, go to step 9.
- Compare actual coolant temperature with scan tool ECT value. If temperatures are within 5°F (3°C) of each other, go to step 9. If temperatures are not within 5°F (3°C) of each other, go to next step.
- Check for high resistance in wiring related to ECT sensor and for poor connections at ECT sensor and PCM. Repair as necessary. After repairs, go to step 11. If no problem is found, go to step 10.
- Repair cooling system as necessary. After repairs, go to step 11.
- Replace ECT sensor. After repairs, go to next step.
- Allow engine to cool completely. Using scan tool, clear DTCs. Turn engine on and allow it to idle. Using scan tool, monitor ENG COOL TEMP. If temperature increases to greater than 70°F (21°C) within 2 minutes, system is okay. If specified temperature is not reached within 2 minutes, go to step 2.
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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.