Skip the auto shop - Our mechanics make house calls

P0686 OBD-II Trouble Code: ECM/PCM Power Relay Control Circuit Low

Check Engine Light

P0686 code definition

The P0686 trouble code detects a voltage error with the ECM/PCM power relay control circuit.

What the P0686 code means

The P0868 code is a standard OBD-II trouble code that notes a voltage error with the relay control circuit that is responsible for supplying the powertrain control module (PCM) and engine control module (ECM) with power. The relays send an output signal to the PCM, and rely on a ground signal, battery voltage signal, and ignition switch input signal. Any time that the PCM notes that the voltage level is abnormal relative to the manufacturer’s recommendations, the P0686 code will be triggered.

What causes the P0686 code?

Many things can cause the P0686 trouble code to be detected:

  • Dead battery
  • Defective circuits
  • Defective PCM relay
  • Malfunctioning PCM
  • Defective battery cables or cable ends
  • Damaged or defective electrical components in the PCM, such as fuses, wires, and connectors

What are the symptoms of the P0686 code?

The P0686 trouble code will usually be accompanied by the Check Engine Light illuminating on the vehicle’s instrument cluster. It is also common for the engine to not start, or not crank.

How does a mechanic diagnose the P0686 code?

The P0686 trouble code should be assessed with the help of a basic OBD-II trouble code scanner. A trained technician will use the freeze frame data from the scanner to observe the code, and to look for additional trouble codes that are stored. Trouble codes should always be cared for in the order that they appear in. the mechanic should reset the trouble codes and restart the vehicle, to see if the code returns. If the P0686 code does not return, then it is likely an intermittent error, or perhaps the result of an erroneous trigger.

If the trouble code returns, then the mechanic should begin with a visual inspection of the electrical components in the PCM. All fuses, wires, and connectors should be inspected, and replaced if damaged. The mechanic should then test to see if the Check Engine Light illuminates when the key is turned to the engine off position. If the light does not illuminate, then the PCM relay is likely the issue, and will need replacement.

If this is not the issue, the mechanic will visually inspect the battery cables and cable ends to make sure that they are all in working order. Next, the charge of the battery will be tested, and a battery load charging system test should take place, with the results being compared to manufacturer’s recommended results. Next, the voltage and ground signals of the applicable circuits will be tested and assessed.

After any repairs, the trouble codes will need to once more be reset, and the vehicle again restarted. The mechanic will then look to see if the P0686 code returns. This allows the mechanic to know as soon as the issue has been resolved.

Common mistakes when diagnosing the P0686 code

The most frequently made mistake when diagnosing the P0686 trouble code is the failure to properly follow the OBD-II trouble code diagnosis protocol. The protocol should be followed step-by-step at all times to guarantee an efficient and thorough inspection and repair.

It is common for the PCM to be replaced erroneously, without even checking the PCM relay, which is a much more likely culprit, and a quicker and more affordable fix.

How serious is the P0686 code?

The P0686 trouble code may not impact the drivability of a vehicle, but it also may leave the car unable to turn on. The code should be addressed as soon as it is detected.

What repairs can fix the P0686 code?

Some possible repairs for the P0686 trouble code are:

Additional comments for consideration regarding the P0686 code

Not all vehicles use a PCM/ECM relay to power the PCM. Some vehicles instead rely on an ignition switch or battery block wire. Vehicles that do not have a PCM/ECM relay will not have a P0686 trouble code.

It is rare for the PCM to need to be replaced, so it should not be considered the issue unless all other options are checked. If the PCM does require replacement, it will need to be reprogrammed.

Need help with a P0686 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.


Post a question and get free advice from our certified mechanics.


More related articles

How to Transfer a Car Title in Idaho
In order to prove ownership of a car, you must have the title. However, when a car is sold, given away or inherited, the title needs...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
Veteran and Military Driver Laws and Benefits in Idaho
The state of Idaho offers a number of benefits and perks for those Americans who have either served in an Armed Forces branch in the...

Related questions

Q: Failure to connect to scan tool

The first thing you should do is use a test light to check for power at the OBD II connector. It is also known as the data link connector or DLC. Only one is fused by the number 12 15A...

Q: Check engine light rough then goes away code cylinder 2 misfire changed plugs wires and cylinder 2 coil driving me nuts

The only things left to check is compression and the condition of the injector if you have changed the plug, wire, and coil. I would go with having the injectors cleaned. It is more likely that you have a clogged...

Q: Codes P2106 and P0720

Hi, thanks for writing in. Code P0720 is for the output speed sensor for the transmission, and code P2016 is for the throttle control actuator in failure mode and may be due to the transmission codes. The system will need...