ECM/PCM Power Relay Control Circuit Low
The P0686 code indicates that the powertrain control module (PCM) has detected a low voltage condition on the control circuit of the ECM/PCM power relay. This relay supplies battery voltage to the ECM/PCM. A low voltage reading suggests a problem with the relay itself, the wiring, or the PCM's ability to control the relay. This can prevent the engine from starting or cause stalling.
01. Most Common Causes
- Faulty ECM/PCM power relay
- Open or shorted wiring in the relay control circuit
- Corroded or loose relay socket connections
- Failed PCM (rare)
02. Symptoms You Will Notice
- Engine cranks but does not start
- Engine stalls shortly after starting
- Check Engine Light illuminated
- No communication with scan tool
03. Step-by-Step Diagnosis
Step 1: Check for blown fuses related to the ECM/PCM relay circuit.
Step 2: Locate the ECM/PCM power relay and test for battery voltage at the relay coil and switch terminals.
Step 3: Inspect the relay control circuit wiring for shorts, opens, or corrosion.
Step 4: Swap the relay with a known good one of the same type and see if the code clears.
Step 5: If the issue persists, test the PCM's ability to ground the relay control circuit using a test light or multimeter.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not safe. The vehicle may stall or fail to start, leaving you stranded. It is best to diagnose and repair immediately.
How much does it cost to fix?▼
Typically $50-$150 for a relay replacement or wiring repair. If the PCM is faulty, costs can exceed $500.