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P0688

ECM/PCM Power Relay Sense Circuit High

ECM/PCM Power Relay Sense Circuit High: the signal voltage is above the expected range. Often an open circuit or a short to supply voltage.

  • Check engine light: Yes
  • Emissions related: Yes
  • Repair difficulty: medium

Symptoms

  • Check engine light on
  • Possible limp mode or no-start
  • Multiple follow-up faults across modules
  • Reset/relearn may be needed after repair

Common causes

Ordered by how often each turns out to be the cause.

  • Main relay contacts stuck closedCommon
  • Sense wire shorted to powerPossible
  • Connector pins corroded - high contact resistanceLess likely

Parts involved

  • ECM
  • Main Relay

P0688 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.