P0418

Circuit Description
The PCM controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the controlled component. Each driver has a fault line which the PCM monitors. When the PCM commands a component ON, the voltage of the control circuit should be low (near 0 volts). When the PCM commands the control circuit to a component OFF, the voltage potential of the circuit should be high (near the battery voltage). If the fault detection circuit senses a voltage other than what the PCM expects, the fault line status changes causing the DTC to set. The relay controls the high current flow to the AIR injection pump and the AIR solenoid. This allows the PCM driver to only have to control the relatively low current used by the relay.

Conditions for Running the DTC
^ The engine speed is greater than 400 RPM.
^ The ignition voltage is between 6.0 volts and 18 volts.

Conditions for Setting the DTC
^ The PCM detects that the commanded state of the driver and the actual state of the control circuit do not match.
^ All the above conditions are present for a minimum of 5.0 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the Malfunction Indicator Lamp on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure records.

Conditions for Clearing the MIL/DTC
^ A last test failed (current DTC) clears when the diagnostic runs and does not fail.
^ A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.
^ Use a scan tool in order to clear the MIL/DTC.

Diagnostic Aids

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

Using Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can aid in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also aid determining how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed.
For an intermittent, refer to Symptoms. Symptom Related Diagnostic Procedures

Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. Listen for an audible click when the relay operates. Command both the ON and the OFF states. Repeat the commands as necessary.
3. This test can detect a partially shorted coil which would cause an excessive current flow. Leaving the circuit energized for 2 minutes allows the coil to warm up. When warm the coil may open (current drops to 0), or short (current goes above 0.75 Amp).
4. The AIR pump relay is located on the AIR pump assembly.
5. Identify and test the relay coil terminals in order to avoid improper diagnosis.
13. If no trouble is found in the control circuit or the connection at the PCM, the PCM may be faulty. However, this is an extremely unlikely failure.
14. The repair is not complete if the scan tool indicates that the diagnostic ran and failed.