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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 270 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)DIAGNOSTIC TESTSDTC P0418: SECONDARY AIR INJECTION RELAY CONTROL CIRCUITDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Turn ignition on, with engine off. Command the secondary Air Injection (AIR) pump and relay on and off with a scan tool. Does the relay turn on and off with each command? If yes, see DIAGNOSTIC AIDS . If no, go to next step.
  3. Turn off the ignition. Remove the AIR relay. Turn ignition on, with engine off. Probe the ignition 1 voltage circuit, coil side, of the AIR relay with a test light that is connected to a good ground. Does the test light illuminate? If yes, go to next step. If no, go to step  12.
  4. Connect a test light between the control circuit of the AIR relay connector and the ignition 1 voltage circuit of the AIR relay connector. Command the AIR pump relay on and off with a scan tool. Does the test light turn on and off with each command? If yes, go to step  11. If no, go to next step.
  5. Does the test light remain illuminated with each command? If yes, go to step  7. If no, go to next step.
  6. Test the control circuit for a short to voltage or an open and repair as necessary. Did you find and correct the condition? If yes, go to step  16. If no, go to step  11.
    7. NOTE: AIR pump may be damaged if control circuit is shorted to ground.
  7. Test the control circuit for a short to ground and repair as necessary. Did you find and correct the condition? If yes, go to next step. If no, go to step  15.
  8. Install the AIR relay. Turn ignition on, with engine off. Command the AIR relay on and off with a scan tool. Does the AIR pump turn on and off with each command? If yes, go to step  16. If no, go to next step.
  9. Replace the AIR fuse as necessary. Disconnect the AIR pump connector. Connect a test light between the AIR supply voltage circuit of the AIR pump connector and the ground circuit of the AIR pump connector. Command the AIR pump relay on and off with a scan tool. Does the test light turn on and off with each command? If yes, go to step  13. If no, go to next step.
  10. Test for an intermittent and for a poor connection at the relay connector and repair as necessary. Did you find and correct the condition? If yes, go to step  16. If no, go to step  14.
  11. Test for an intermittent and for a poor connection at PCM. Did you find and correct the condition? If yes, go to step  16. If no, go to step  15.
  12. Repair open or short to ground in ignition 1 voltage circuit. After repairs, go to step  16.
  13. Replace AIR pump. After repairs, go to step  16.
  14. Replace AIR relay. After repairs, go to step  16.
  15. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  16. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  17. Using scan tool, observe stored information. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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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.