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 CONTROL SYSTEM - TESTS W/CODES - DTC P0352-P0452DTC P0418DTC P0418: SECONDARY AIR INJECTION (AIR) PUMP RELAY CONTROL CIRCUIT (4.8L, 5.3L & 6.0L "C" & "K" SERIES)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 4.3 W, Standard
Diagnostic Procedures
1999 GMC Forward Control 4.3 W, StandardSECTION 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.
- Turn ignition on, with engine off. Using scan tool, command AIR pump relay on and off. If relay turns on and off as commanded, go to next step. If relay does not turn on or off when commanded, go to step 5.
- Turn ignition off. Disconnect PCM harness connector. Install a fused jumper to control circuit at PCM harness connector. Turn ignition on, with engine off. Using a DVOM, check current from relay control circuit in PCM harness connector to ground for 2 minutes. If current draw is less than 0.75 amp (but not zero), see DIAGNOSTIC AIDS . If current draw is not less than 0.75 amp, go to next step.
- Turn ignition off. Remove AIR pump relay. Check resistance from relay control circuit in PCM harness connector to ground. If resistance is infinite, go to step 12. If resistance is not infinite, go to step 10.
- Turn ignition off. Remove AIR pump relay. Connect test light between relay coil terminals in relay harness connector. Turn ignition on. Using scan tool, command relay on and off. If test light turns on and off with each command, go to step 8. If test light does not turn on and off with each command, go to next step.
- Connect test light to ground and probe ignition feed circuit in AIR pump relay harness connector. If test light illuminates, go to next step. If test light does not illuminate, go to step 11.
- Turn ignition off. Reinstall AIR pump relay. Disconnect PCM harness connector. Turn ignition on. Momentarily connect a fused jumper wire between ground and PCM harness connector relay control circuit. If relay turns on when circuit is grounded and off when circuit is opened, go to step 9. If relay does not turn on when circuit is grounded and off when circuit is opened, go to step 10.
- Check connections at AIR pump relay. Repair as necessary. After repairs, go to step 14. If relay connection is okay, go to step 12.
- Check connections at PCM. Repair as necessary. After repairs, go to step 14. If connections are okay, go to step 13.
- Repair faulty relay control circuit. After repairs, go to step 14.
- Repair faulty ignition feed circuit. After repairs, go to step 14.
- Replace AIR pump relay. After replacing relay, go to step 14.
- Replace PCM. Program replacement PCM using required equipment. After repair is completed, go to next step.
- Using scan tool, select DTC, CLEAR INFO function. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter this DTC. Operate vehicle within conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2.
- Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTCs are displayed, go to applicable DTC test.
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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.