What Does P0686 Mean?
The PCM has detected a low voltage condition on the ECM/PCM power relay control circuit. This means the wire that commands the main PCM power relay is being pulled to ground when it should not be, or the relay coil has an internal short. This can cause the relay to remain energized continuously or behave erratically, potentially leading to key-off power drain or inability to properly shut down the engine.
Safety Warning
This condition can cause unpredictable PCM behavior, including sudden engine stall or inability to start. A continuously energized relay can also drain the battery when the vehicle is parked. Repair immediately to prevent being stranded or experiencing a stall in traffic.
Common Causes
35%
Short to ground in the relay control wire between PCM and relay
25%
ECM/PCM power relay coil shorted internally
20%
Corroded connector at relay socket causing low-resistance path to ground
10%
PCM relay driver transistor failed low
10%
Water intrusion in the fuse box or relay panel
Diagnostic Steps
1
Disconnect the relay and measure resistance of the control circuit wire from the PCM to the relay socket; check for any short to ground (should read OL to ground).
2
Measure the relay coil resistance; it should be 60-100 ohms. A very low reading indicates an internal short in the coil.
3
Inspect the relay socket and surrounding fuse box area for moisture, corrosion, or evidence of water intrusion that could create ground paths.
4
With the relay removed, measure voltage at the PCM relay driver output pin; it should be at or near battery voltage when the key is on. A low reading with the load disconnected points to a PCM issue.
5
Inspect the full length of the relay control wire for chafing against metal brackets, bolts, or sharp edges that could expose bare wire to ground.
Estimated Repair Cost
$50 - $500
Parts + labor, varies by vehicle and location
The PCM has detected a low voltage condition on the ECM/PCM power relay control circuit. This means the wire that commands the main PCM power relay is being pulled to ground when it should not be, or the relay coil has an internal short. This can cause the relay to remain energized continuously or b...
The most common cause of P0686 (ECM/PCM Power Relay Control Circuit Low) is: Short to ground in the relay control wire between PCM and relay
Typical repair costs for P0686 range from $50 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This condition can cause unpredictable PCM behavior, including sudden engine stall or inability to start. A continuously energized relay can also drain the battery when the vehicle is parked. Repair immediately to prevent being stranded or experiencing a stall in traffic.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0686 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Engine Control Module Power Supply
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
