Code Definition

P0686 — Control Module Power Relay “B” Control Circuit/Open

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

About P0686

P0686 is a generic (obd-ii standard) powertrain diagnostic trouble code recorded for Holden vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0686 is stored, the module has detected an open (broken or disconnected) electrical path in the monitored circuit.

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 P0686 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 engine control module (ECM/PCM) energises its Power Relay B to supply ignition on and run voltage to a designated group of circuits, and it monitors the relay coil control circuit for proper voltage or current feedback. The PCM drives the relay coil by switching ground or supply to it, confirming the relay responds to its command. DTC P0686 is set when the PCM commands Relay B on but detects an open circuit, excessive resistance, or no coil current within the specified timeout, indicating the control circuit is open or cannot be controlled. Depending on which circuits Relay B feeds, the fa...

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