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 MANUALSGMC1991FORWARD CONTROL 5.7 K, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROLS - TESTS W/CODES - GASOLINETROUBLE CODE CHARTSCODE 55, ECM ERROR (EXCEPT 2.5L)
1991 GMC Forward Control 5.7 K, Automatic
Code 55, ECM Error (Except 2.5L)
1991 GMC Forward Control 5.7 K, AutomaticSECTION Code 55, ECM Error (Except 2.5L)
Ensure ECM grounds are good and MEM-CAL is properly latched. If okay, replace ECM. Clear codes and confirm closed loop operation. Check operation of "SERVICE ENGINE SOON" light.
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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.