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P0688ECM/PCM Power Relay Sense Circuit/Open

The engine computer commanded its main power relay on but sees no voltage coming back on the relay's sense circuit, meaning switched power is not being delivered.

Definition

After energizing the main power relay, the PCM verifies the result by monitoring a sense (feedback) circuit wired to the relay's switched output. P0688 sets when that sense circuit reads open or dead even though the relay is commanded on — the relay contacts, an output fuse, or the sense wiring have failed. Because the relay output feeds the PCM's power inputs and often the injectors and ignition system, symptoms include crank-no-start, sudden stalling, intermittent shutdowns over bumps, and a MIL with possible companion codes from unpowered modules. The stall-while-driving potential makes this a safety-relevant repair.

Common causes

  • Failed main power relay contacts (coil works but output is dead)
  • Blown fuse on the relay's switched output circuit
  • Open sense wire between the relay output and the PCM
  • Corroded or heat-damaged fuse block terminals
  • Failed PCM sense input (rare)

Common misdiagnoses

  • PCM replacement when the relay contacts or a fuse are at fault
  • Battery or starter replacement for the no-start
  • Ignition switch replacement
  • Fuel pump diagnosis for the sudden-stall complaint

Troubleshooting steps

  1. 1.Check output-side fuses

    Test every fuse fed by the ECM/PCM relay output with a meter, since one blown fuse can kill the sense feed.

  2. 2.Verify relay output

    With the relay commanded on, measure the switched output terminal at the socket — battery voltage should be present; if not, replace the relay.

  3. 3.Test the sense wire

    Confirm continuity from the relay output (or its fused branch) to the PCM sense pin, and check for corrosion at splices.

  4. 4.Load-test the circuit

    Use a headlamp bulb or loaded test to confirm the relay output can carry current, since burned contacts can pass a voltmeter check but fail under load.

  5. 5.Wiggle-test for intermittents

    Flex the fuse block harness while monitoring the sense voltage to catch a connection that opens with vibration.

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