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DTC P0508: IAC Valve Sense Circuit Low: Testing

  1. Start the engine. Allow the engine to idle. If the engine will not idle, maintain an engine speed between 800 and 1500 RPM. With the DRBIII®, read the IAC Current. Is the IAC Current below 146 mA? If yes, go to next step. If no, go to step  8.
  2. Turn the ignition off. Disconnect the IAC Motor harness connector. Remove the IAC Motor. Inspect the IAC air passages for restriction and damage to the IAC valve. Measure the resistance across the IAC Motor terminals (component side). Is the resistance 8.7-10.7 ohms? If yes, go to next step. If no, replace the IAC Motor.
    3. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal-to-pin connection. Use Miller Special Tool (8815) when performing diagnosis at PCM harness connectors.
  3. Ensure the ignition is off. Disconnect the PCM harness connectors. Measure the resistance of the IAC Motor Sense circuit from the IAC Motor harness connector to the appropriate terminal at Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open in the IAC Sense circuit.
  4. Measure the resistance between ground and the IAC Motor Sense circuit in the IAC Motor harness connector. Is the resistance below 100 ohms? If yes, repair the short to ground in the IAC Motor Sense circuit. If no, go to next step.
  5. Measure the resistance between ground and the IAC Motor Driver circuit in the IAC Motor harness connector. Is the resistance below 100 ohms? If yes, repair the short to ground in the IAC Motor Driver circuit. If no, go to next step.
    6. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal-to-pin connection. Use Miller Special Tool (8815) when performing diagnosis at PCM harness connectors.
  6. Measure the resistance of the IAC Motor Driver circuit from the IAC Motor harness connector to the appropriate terminal at Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open in the IAC Driver circuit.
    7. NOTE: Before continuing, check the PCM harness connectors for corrosion, damage or terminal push out. Repair as necessary.
  7. There are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
    8. WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.
  8. The conditions that set the DTC are not present at this time. The following may help in identifying the intermittent condition. With the engine running at normal operating temperature, monitor the DRBIII® parameters related to the DTC while wiggling the wiring harness. Look for parameter values to change and/or a DTC to set. Review the DRBIII® Freeze Frame information. If possible, try to duplicate the conditions under which the DTC was set. Refer to any Technical Service Bulletins (TSB) that may apply. Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or partially broken wires. Visually inspect the related wiring harness connectors. Look for broken, bent, pushed out, or corroded terminals. Were any of the above conditions present? If yes, repair as necessary. If no, test is complete.
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