Code Definition

P0688 — PCM Power Relay Control Circuit High

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Jeep

About P0688

P0688 is a generic (obd-ii standard) powertrain diagnostic trouble code recorded for Jeep vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0688 is stored, the module has detected a signal or supply voltage higher than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0688 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Powertrain Control Module (PCM) monitors its power relay control circuit to confirm proper voltage and relay operation during key on and key off transitions. DTC P0688 is set when the PCM detects a voltage level higher than the expected range on the relay control circuit while the relay is commanded off. This condition indicates the control circuit may be shorted to battery voltage, preventing the PCM from correctly switching the relay state and potentially keeping the PCM or related circuits powered unintentionally. The PCM drives this relay to supply itself and engine systems, so a high ...

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