OBD-II trouble code
P0686: ECM/PCM Power Relay Control Circuit Low
The relay control circuit is reading low, which usually means it is shorted to ground. The counterintuitive consequence is a relay stuck on rather than off — so the common complaint is a battery that keeps going flat, not a no-start.
Quick facts
- System
- Powertrain
- Category
- PCM / Electronics
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $30 – $900
- DIY difficulty
- Intermediate DIY
What does P0686 mean?
Most people read a power relay code and expect a car that will not start. P0686 frequently produces the opposite complaint, and understanding why is the whole diagnosis.
The powertrain control module switches its main relay by grounding one end of the relay coil. Ground the coil and the relay pulls in; release it and the relay drops out. Low voltage on that circuit therefore does not mean the relay lost its command — it means something is holding the circuit at ground when the module is not asking for it. A relay held at ground is a relay held ON.
So the classic P0686 vehicle drives perfectly. It starts, it runs, nothing feels wrong. Then it sits overnight and the battery is flat in the morning, because the main relay never dropped out and the module and everything it feeds stayed awake in the driveway. Owners chase the battery, chase the alternator, sometimes buy both, and the code that explains it has been stored the entire time.
That is also why this page rates as drivable while its siblings do not. Nothing about a permanently energised main relay creates a hazard while the vehicle is moving. What it does is strand you where you parked, which is a real cost but a different kind of one.
The short to ground is usually somewhere the control wire has lost its insulation against a metal edge, a bracket, or a bolt shank — often at a point where the harness passes through a bulkhead or is clipped tight to the body. Water in the underhood fuse box does the same job more quietly, bridging the control terminal to the box's ground through a film of contaminated moisture that comes and goes with the weather.
One test cleanly separates a wiring short from a failed driver inside the module: unplug the relay and check whether the control circuit still reads low. Still grounded with the relay out means the fault is in the wire between the module and the socket, or inside the module. Reading normally with the relay out means the relay's own coil has failed internally and is grounding the circuit through itself.
Common causes
- Control wire shorted to ground through chafed or melted insulation
- Water or contamination in the underhood fuse box bridging the control terminal to ground
- Relay coil failed internally and grounding the control circuit through itself
- Harness damage where the wire passes a bracket, bulkhead grommet or sharp body edge
- Failed low-side driver inside the PCM stuck in the on state
- Damage from a previous repair, such as a pierced wire that was never sealed
Symptoms
- Battery flat after the vehicle sits overnight or for a day or two
- Cooling fans, fuel pump or relay audibly staying on after the key is removed
- Engine runs and drives normally with no performance complaint at all
- Check engine light with P0686 stored
- Repeated battery or alternator replacement that did not solve the drain
- Underhood relay noticeably warm long after the vehicle was shut off
Diagnostic steps
- 1.After shutting the vehicle off and waiting for the normal module sleep period, listen and feel for the main relay still being energised. A relay that is still pulled in confirms the failure mode immediately.
- 2.Perform a parasitic draw test. A vehicle that will not settle to its normal resting current after the sleep interval is telling you the relay never released.
- 3.Pull the main relay and see whether the parasitic draw disappears — that isolates the drain to this circuit rather than to an accessory.
- 4.With the relay out, measure the control circuit to ground. Still low means the short is in the wire or in the module; normal means the relay coil itself was the path.
- 5.Disconnect the PCM connector and re-measure the wire to ground. A short that persists with both ends disconnected is in the harness and nowhere else.
- 6.Inspect the harness where it crosses brackets, bulkheads and body edges, and check the underhood fuse box for water intrusion and corrosion.
- 7.If the wire is clean end to end and the relay is good, the module's driver is the remaining candidate.
Repair cost
$30 – $900
A relay replacement is $15-$60. Locating and repairing a short to ground is usually $150-$400, and the labor is in the finding rather than the fixing. Budget for a battery too — a battery that has been deeply discharged several times by this fault often will not recover, so a $120-$250 replacement is a realistic part of the total. PCM replacement with programming is the $600-$1,500 outlier.
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.