OBD-II trouble code
P0685: ECM/PCM Power Relay Control Circuit / Open
The wire the engine computer uses to switch on its own power relay has gone open. Because the module commands the relay that keeps it alive, this fault can take the whole engine down without warning.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $30 – $900
- DIY difficulty
- Intermediate DIY
What does P0685 mean?
There is a loop at the centre of this code that makes it unlike any other circuit fault on the vehicle. The powertrain control module (PCM) does not simply receive power — it commands the relay that supplies it. Turn the key and the ignition switch wakes the module through a brief initial feed; the module then energises the main power relay, and from that moment the relay is holding up the module that is holding the relay closed. The engine runs inside that loop.
P0685 means the control wire from the module to the relay coil has gone open. The command cannot reach the relay.
That loop is also why the failure can be so abrupt. Break the control wire while the engine is running and the relay drops out, the module loses its supply, and injectors, ignition and fuel pump go dead in the same instant. This is not a warning light that gives you a week — it is a code that can end with the engine stopping in traffic with no power assist for the steering or brakes. That is the reason for the rating on this page and the reason a P0685 that has stalled a vehicle even once should be repaired before it is driven again.
There is a practical wrinkle worth knowing before you plug in a scan tool. A module without power cannot answer a scan tool either, so on a vehicle that is currently dead the fault often reads as a complete failure to communicate rather than as a stored code. The code becomes visible once power is restored — after the relay is jumped, or after an intermittent fault closes itself again. When it does appear, the freeze frame is unusually valuable, because it captured the conditions at the moment the module lost its own supply and that is information you cannot recreate on the bench.
Open is also the most literal of the three control-circuit codes and the most mechanically satisfying to find. The break is somewhere in a single, short, low-current run of wire between one PCM pin and one relay socket terminal. It is a defined path with two ends, which is the best situation an electrical diagnosis can offer.
Common causes
- Open or broken wire between the PCM and the main power relay coil
- Corroded, spread, or backed-out terminal in the relay socket
- Relay coil open internally, so no circuit exists for the module to complete
- Damaged or chafed harness section where it passes a bracket or the engine
- Corrosion in the underhood fuse box, particularly after water intrusion
- Failed low-side driver inside the PCM, which is the least likely cause and should be proven last
Symptoms
- Engine stalls suddenly while driving, with no warning and no stumble first
- No-start with no crank, no fuel pump prime, and no dash warning lights
- Scan tool cannot communicate with the engine module at all
- Intermittent total electrical dropout that recovers on its own
- Check engine light stored with P0685 after power has been restored
- Vehicle sometimes starts after tapping the underhood relay box
Diagnostic steps
- 1.Treat the vehicle as unsafe to drive until the fault is found. A code that has already stalled the engine will do it again.
- 2.Pull the freeze frame data before clearing anything — it records what the module saw as it lost power and cannot be reproduced later.
- 3.Locate the main power relay in the underhood fuse box and swap in an identical relay from a non-critical circuit as an initial check.
- 4.With the relay removed, identify the two coil terminals and check for continuity from the control terminal back to the PCM pin. An open here is the fault, found.
- 5.Inspect the relay socket terminals for spreading, green corrosion and heat discoloration; a socket that has been running warm has been running resistive.
- 6.Check the fuse box for water intrusion. Underhood boxes with a failed seal corrode from inside and the damage is not visible until the box is opened.
- 7.Wiggle-test the harness between the fuse box and the PCM with the circuit live, since an intermittent open shows itself under movement and not at rest.
- 8.Only after the wire, socket and relay have all been proven good should the PCM's driver be suspected.
Repair cost
$30 – $900
A relay is typically $15-$60 and is the cheapest possible outcome. A repaired wire or reterminated relay socket usually falls in the $100-$300 range with diagnostic time. A corroded underhood fuse box can run $250-$700. PCM replacement with programming is the expensive tail at $600-$1,500 and is rarely the actual fault — verify it rather than assuming it.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with engine main / powertrain relay replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.