P0685: ECM/PCM Power Relay Control Circuit Open

What does P0685 actually mean?

The main relay that keeps the engine computer powered up after the key is turned off, used for tasks like running the cooling fan or finishing a self test, has an open circuit on its control side.

The code sits in the main power relay circuit area and is reported by the comprehensive component monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Engine cranks but will not start
• Engine starts and immediately stalls
• Cooling fan does not run after shutoff when it should
• Intermittent no-start

Why does P0685 set?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a main power relay circuit fault becoming an expensive guess.

Corroded relay socket or connector is the one to rule out before anything else. The most common and cheapest cause to check first.

The order worth checking on P0685

Start with freeze frame. There is no useful engine freeze frame here, check whether the fault occurred at key on, during cranking or right after starting. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

Diagnosing this one properly means commanding a component and watching whether it responds, which a read-only tool cannot do at any price. Bidirectional control of the main relay circuit is needed to confirm whether the relay or the computer driver circuit is at fault.

The order worth working in

• 1. Corroded relay socket or connector: The most common and cheapest cause to check first.
• 2. Blown fuse: A cheap and common cause to check next.
• 3. Sticking or failed main relay: The relay contacts wear out over time and can stick or fail open.
• 4. Wiring fault between the computer and relay: A break in this wiring prevents proper relay control.
• 5. Failed driver circuit inside the computer: Least likely, but the output stage controlling the relay can fail.

Is it safe to drive with P0685?

This usually shows up as a no-start or a stall right after starting rather than a problem while already driving, so if the engine dies immediately after starting, do not keep trying to force it.

This is a severity 2 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0685?

An intermittent power relay fault can turn into a complete no-start with little warning.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.