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OBD-II fault code definitions, symptoms, causes, and step-by-step diagnostics.
Standardized generic DTCComputer & output circuits

P0688 — ECM/PCM Power Relay Sense Circuit/Open

P0688 is the standardized powertrain DTC for “ECM/PCM Power Relay Sense Circuit/Open.” The code identifies the monitored condition; it does not by itself prove which part failed.

Standardized generic definition
ECM/PCM Power Relay Sense Circuit/Open
DTC category
Computer & output circuits
Diagnostic priority
Moderate to high depending on symptoms. Module codes are often secondary to voltage, network, connector, or calibration problems.
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What P0688 means

P0688 is the standardized powertrain DTC for “ECM/PCM Power Relay Sense Circuit/Open.” The code identifies the monitored condition; it does not by itself prove which part failed.

Start with the stored data and the simplest external checks before replacing the component named in the code.

How to interpret the code wording

“Circuit/Open” points first toward circuit integrity: connector pin fit, broken wiring, missing power/ground, excessive resistance, or an open load/actuator. Confirm voltage and voltage drop under load before condemning a control module.

Possible symptoms

  • Multiple warning lights
  • Intermittent no-start or limp mode
  • Communication loss
  • Unexpected module behavior

Causes to test

These are diagnostic possibilities, not a statistical parts-replacement ranking. Vehicle-specific service information takes priority.

  • Low/unstable system voltage
  • Power/ground or connector problem
  • CAN/network wiring fault
  • Incorrect or incomplete programming/calibration
  • Internal module fault

Step-by-step diagnostic workflow

  1. Stabilize and test battery/charging voltage before module replacement.
  2. Perform a full-vehicle module scan and identify which modules communicate.
  3. Check module power, grounds, fuses, and network termination/wiring.
  4. Review programming/calibration status and applicable service information.
  5. Replace/reprogram a module only after external causes are ruled out.
  6. Clear codes only after repair, then complete an appropriate drive cycle and verify the code and symptoms do not return.
Preserve evidence: EPA OBD guidance emphasizes that stored DTCs and freeze-frame data are important to proper diagnosis. Save them before clearing codes.

Scan data worth checking

  • Full module scan
  • System voltage
  • Network DTCs
  • Module software/calibration identifiers where supported

Severity and drivability

Moderate to high depending on symptoms. Module codes are often secondary to voltage, network, connector, or calibration problems.

Stop driving and investigate immediately if there is a flashing MIL, oil-pressure warning, overheating, fuel leakage, severe misfire, major loss of power, abnormal mechanical noise, or a transmission that is slipping badly.

Common diagnostic mistakes

  • Replacing a PCM/TCM based on a single internal code without power/ground checks
  • Ignoring low-voltage history
  • Programming a module without a stable power supply

Related generic codes

P0687 — ECM/PCM Power Relay Control Circuit HighP0689 — ECM/PCM Power Relay Sense Circuit LowP0686 — ECM/PCM Power Relay Control Circuit LowP0690 — ECM/PCM Power Relay Sense Circuit HighP0685 — ECM/PCM Power Relay Control Circuit/OpenP0644 — Driver Display Serial Communication Circuit

Source and accuracy notes

Generic DTC structure and standardized definitions are governed by SAE J2012/J2012DA. Actual enable criteria, thresholds, circuit pinouts, test values, software conditions, and repair procedures vary by vehicle. Use current OEM service information for the exact year/engine/calibration and check applicable manufacturer technical service bulletins. See the site methodology and primary-reference policy.