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 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS TESTS W/CODES - DIESELSELF-DIAGNOSTIC SYSTEMHARD OR INTERMITTENT TROUBLE CODE DETERMINATION
1999 GMC Forward Control 4.3 W, Standard
Hard Or Intermittent Trouble Code Determination
1999 GMC Forward Control 4.3 W, StandardSECTION Hard Or Intermittent Trouble Code Determination
NOTE:
DTCs will be recorded at various operating times. Some codes require operation of related sensor or switch for 5 seconds; others require operation for 5 minutes or longer at normal operating temperature, vehicle speed and load. Therefore, some DTCs may not set in a service bay operational mode and may require road testing vehicle in order to duplicate conditions under which code will set.
During any diagnostic procedure, determine if DTC(s) are hard failure codes or intermittent failure codes. Diagnostic procedures will not always help analyze intermittent codes. To determine hard codes and intermittent codes:
- Enter diagnostic mode. Read and record all stored DTCs. Exit diagnostic mode, and clear DTCs. See CLEARING DTCS .
- Apply parking brake, and place transmission in Neutral or Park. Block drive wheels, and start engine. MIL should turn off. Operate warm engine at specified RPM for 2 minutes and note MIL.
- If MIL illuminates, enter diagnostic mode. Read and record DTCs. This will reveal hard failure codes. Oxygen sensor related codes may require a road test to reset hard failure after DTCs were cleared.
- If MIL does not illuminate, all stored DTCs were intermittent failures, except as noted above.
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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.