Code Definition

P0691 — ECM/PCM Power Relay Sense Circuit Low

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

About P0691

P0691 is a generic (obd-ii standard) powertrain diagnostic trouble code recorded for Ford vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0691 is stored, the module has detected a signal or supply voltage lower 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 P0691 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

When the engine control module (ECM/PCM) energises its power relay, it monitors the relay sense circuit voltage to verify the relay contacts have closed and are supplying battery voltage on the switched side as expected. This sense feedback confirms the relay actually powered up the circuits it feeds. DTC P0691 is set when the PCM detects a sense voltage significantly below battery level while the relay is commanded on, suggesting an open circuit, blown fuse, or relay contact failure, and the condition lasts beyond a predetermined time. Because the relay supplies the PCM and engine systems, a ...

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