P0686 indicates the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the control circuit for its main power relay is reading lower than expected voltage. This circuit provides battery power to the PCM when the ignition is on. The code is set when the module monitors the voltage on this circuit and finds it below the normal threshold.
The PCM power relay (often called the main relay or EFI relay) has failed internally, preventing proper voltage from reaching the control module. This is the most common cause.
The electrical connector to the main relay may be corroded, oxidized, or have damaged pins, causing high resistance and low voltage on the control circuit.
A fuse protecting the PCM power relay circuit has blown due to a short or overload, cutting voltage to the control circuit.
The wiring harness for the main relay circuit may be damaged, chafed, or shorted to ground, pulling the voltage low.
A weak or corroded ground connection at the PCM or relay can cause voltage drop and low circuit readings.
The PCM itself may have an internal electrical fault affecting its ability to control or monitor the relay circuit. This is the least likely cause and should be diagnosed only after all other possibilities are ruled out.
No. While the PCM can fail, the power relay itself is the most common cause. Many technicians report that replacing the PCM for this code is unnecessary in the majority of cases.
Severely low battery voltage can contribute to this code setting, but it typically will not cause the code alone. A weak battery may prevent the relay from engaging properly, which could trigger the code.
It is not recommended. The vehicle may not start, may stall unexpectedly, or may not run properly. Driving with this code risks being stranded.
Costs vary significantly by vehicle. The main relay itself is often $20-$100, while diagnostic labor and any wiring repairs add to the cost. PCM replacement, when truly necessary, can be much more expensive and typically requires programming.