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P0689ECM/PCM Power Relay Sense Circuit Low

The voltage on the main power relay's sense circuit is lower than expected while the relay is commanded on, indicating weak or missing switched power to the engine computer.

Definition

The PCM monitors a sense circuit connected to the switched output of its main power relay to confirm the relay actually delivered power. P0689 sets when that sense voltage is below the expected threshold with the relay commanded on — from high resistance in the relay contacts or fuse block, a partially shorted sense circuit, or a blown output fuse. Symptoms include intermittent stalling, extended cranking or no-start, flickering warning lamps, erratic behavior from relay-fed circuits, and a MIL; on several GM applications this code traces to corroded underhood fuse block terminals. Loss of relay output while driving can stall the engine without warning.

Common causes

  • Corroded or heat-damaged fuse block/relay socket terminals causing voltage drop
  • Worn main power relay with high-resistance contacts
  • Blown or partially failed fuse on the relay output branch
  • Sense wire chafed and shorted to ground
  • Poor battery cable or ground connections lowering the whole supply
  • Failed PCM sense input (rare)

Common misdiagnoses

  • PCM replacement when the fuse block or relay is the real fault
  • Battery replacement for low-voltage symptoms
  • Alternator replacement for flickering electrical behavior
  • Fuel system diagnosis for the intermittent stall

Troubleshooting steps

  1. 1.Measure the sense voltage under load

    With the engine running, the sense circuit should sit within about 0.5 V of battery voltage; a bigger gap confirms resistance in the feed path.

  2. 2.Voltage-drop test the relay output path

    Measure the drop across the relay contacts and each fuse in the branch while loaded; more than about 0.2 V across any single connection is excessive.

  3. 3.Inspect the fuse block

    Pull the relay and fuses and check the cavities for green corrosion, heat discoloration, and loose terminal tension.

  4. 4.Check the sense wire for a partial short

    Disconnect both ends and confirm the wire is not leaking to ground and has near-zero end-to-end resistance.

  5. 5.Verify battery and ground connections

    Clean and torque the battery terminals and main engine/body grounds, then retest before condemning any module.

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