P0686
UniversalECM/PCM Power Relay Control Circuit Low
High SeverityThe Engine Control Module (ECM) or Powertrain Control Module (PCM) relies on a power relay to receive constant voltage for operation. This code indicates that the control circuit for this relay is experiencing a voltage that is lower than specifications, which can prevent the module from powering on correctly and lead to serious operational issues.
⚠️ Before You Begin (Safety First)
Always disconnect the battery before working on electrical components. Be cautious when testing electrical circuits to avoid short circuits or damaging sensitive electronic components. An engine that stalls unexpectedly or fails to start can create a significant safety hazard.
Common Symptoms
- Engine cranks but does not start
- Engine stalls unexpectedly while driving
- No communication with the ECM/PCM via an OBD-II scanner
- Check Engine Light (CEL) illuminated
- Random engine misfires or rough idle (less common, usually if power is intermittent)
Common Causes
- Faulty ECM/PCM power relay
- Blown fuse for the ECM/PCM power relay or its circuit
- Open circuit in the control side of the power relay wiring
- Short to ground in the control circuit wiring
- Corroded, loose, or damaged electrical connections at the relay or ECM/PCM
- Faulty ECM/PCM (rare, usually a last resort diagnosis)
Frequently reported on: General/Universal
Diagnostic Steps
- Locate and inspect all fuses related to the ECM/PCM and its power relay, typically found in the underhood fuse/relay box. Replace any blown fuses.
- Identify the ECM/PCM power relay. You can often swap it with a known good relay of the same type (e.g., horn relay if identical) to test if the relay is faulty.
- Visually inspect the wiring and connectors leading to the ECM/PCM power relay and the ECM/PCM itself for any signs of damage, corrosion, or loose connections. Repair as necessary.
- Using a multimeter, test the continuity and voltage at the relay socket and along the control circuit wires, following your vehicle's specific wiring diagram if available.
- If the relay and fuses are good, and wiring appears intact, further diagnose for an open circuit or short to ground in the control wire leading to the relay.
👨🔧 When to Call a Professional
If after checking fuses and replacing the relay the problem persists, or if you are uncomfortable testing complex electrical circuits and wiring, it's best to consult a professional mechanic. There could be a more complex wiring issue, a faulty ground, or even a problem with the ECM/PCM itself.
Commonly Asked Questions
What does 'ECM/PCM Power Relay Control Circuit Low' mean?
It means the circuit that controls the power relay for your vehicle's main computer (ECM/PCM) is experiencing a voltage lower than expected, which can prevent the computer from receiving proper power.
Can a bad power relay cause my car not to start?
Yes, absolutely. If the ECM/PCM power relay fails or its control circuit is low, the engine's computer won't power on, preventing the engine from starting or causing it to stall.
Where is the ECM/PCM power relay located?
The relay is typically located in one of the vehicle's fuse/relay boxes, most commonly under the hood. Consult your owner's manual or a vehicle-specific service manual for its exact location.
Is it safe to drive with a P0686 code?
No, it is not safe. This code indicates a critical power supply issue to the engine's computer, which can lead to unexpected engine stalling or a no-start condition, posing a significant safety risk.
What tools do I need to diagnose a P0686 code?
A basic multimeter is essential for testing voltage and continuity in the circuit. A fuse puller and possibly a wiring diagram for your specific vehicle will also be very helpful.