GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Holden 🢒 2007 🢒 WM Sedan 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P0688

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category

DTC Descriptor

DTC P0688: Engine Control Ignition Relay Feedback Circuit Malfunction

Circuit/System Description

The engine control module (ECM) grounds the Engine Controls Ignition Relay FeedBack circuit through an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line that is monitored by the ECM. When the ECM is commanding a component ON, the voltage potential of the control circuit should be low, near 0 volts. When the ECM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be high, near battery voltage. If the ECM detects that the actual state of the control circuit does not match the expected state, a DTC will set.

Conditions for Running the DTC
    • The ignition is ON.
    • The system voltage is between 9-18 volts.

Conditions for Setting the DTC
    • An incorrect voltage level has been detected on the Engine Controls Ignition Relay FeedBack circuit.
    • The condition must be present for a minimum of 30 seconds.

Action Taken when the DTC Sets

DTC P0688 is a type B DTC.

Conditions for Clearing the MIL/DTC

DTC P0688 is a type B DTC.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs
    •  Charging System Test

Scan Tool Reference

Control Module References

Circuit/System Verification

Ignition ON, command the relay ON and OFF with a scan tool. Feel or listen to verify that the relay turns ON and OFF with each command.

Circuit/System Testing
  1. Ignition OFF, disconnect the Engine Controls Ignition Relay.

    2. Important: The test lamp is used to load the circuit and may not illuminate.

  2. Connect a test lamp between the B+ supply circuit of the main relay and a known ground.
  3. Ignition ON, test for less than 0.1 volts difference between the actual battery voltage and the B+ supply voltage of the main relay.
    4. ⇒If greater than 0.1 volts, test the B+ voltage circuit of the main relay for a short to ground or an open/high resistance.
  4. Ignition ON, connect a test lamp between the control circuit supply voltage and a known ground.
    5. ⇒If test lamp does not illuminate, test the control circuit for an open/high resistance or short to ground. If all circuits test normal replace the ECM.
  5. Ignition OFF, connect a test lamp between the control circuit of the main relay and the earth circuit of the relay coil at the underhood electrical centre.
  6. Ignition ON. The test lamp should illuminate when the ignition switch is cycled ON and OFF.
    7. ⇒If the test lamp does not illuminate, test the earth circuit of the relay coil for open/high resistance.
  7. If all circuits/connections test normal, test or replace the main relay.

Repair Instructions

Perform the Diagnostic Repair Verification .

Repair Verification

Ignition ON, command the relay ON and OFF with a scan tool. Feel or listen to verify that the relay turns ON and OFF with each command.