P0685: ECM/PCM power relay control circuit open

What the code tells you on its own

• System. Powertrain. Engine, fuel, air, ignition, emissions and transmission. The overwhelming majority of what workshops bring to the AI.
• Functional group. Glow plugs and preheat
• Subsystem. Computer output circuit and ECU

What we see when P0685 turns up

Measured across the diagnostics run through Wrenchlane by independent workshops, not taken from a reference table. Sightings span March 2026 to May 2026.

• 8 diagnostics carried this code.
• 3 separate workshops met it, so this is not one shop seeing the same fault repeatedly.
• 25% of the time it arrived with no symptom description at all, meaning the technician had nothing but the code to work from.
• Most often on Citroen, which is where we have seen it most rather than a claim about the fleet at large.

What this page does not tell you

A code names a symptom the car noticed, not the part that failed. P0685 on one vehicle and the same code on another can have different causes, and the counts above describe what independent workshops brought to us rather than how common the fault is across all cars. Getting from the code to the cause needs the vehicle: its make, model, engine and what it is actually doing.

Run P0685 against the car in front of you. Enter the code and the symptoms and get ranked probable causes for that specific vehicle, with the checks in the order worth doing them. Free account, no card.

Other glow plugs and preheat codes

• P0675, Cylinder 5 glow plug circuit
• P0684, Glow plug control module to PCM communication circuit range/performance
• P0380, Glow plug or heater circuit A
• P0671, Cylinder 1 glow plug circuit
• P0672, Cylinder 2 glow plug circuit
• P0683, Glow plug control module to PCM communication circuit