Skip the auto shop - Our mechanics make house calls

OBD-II Trouble Code P0688: ECM/PCM Power Relay Sense Circuit/Open

diagnostic check engine light

P0688 code definition

The P0688 code is a generic powertrain code related to output circuit issues, including internal computer failures. In this case, the engine control module (ECM), also known as a powertrain control module (PCM) has signaled too much power to the power relay sense circuit, which powers the: ECM/PCM.

What the P0688 code means

A code P0688 will be stored when the relay control circuit that supplies the ECM with power sends out a high or abnormal voltage reading.

Vehicles that use a PCM/EC relay to supply the ECM with power and ground signals will be the only type of vehicles in which this code will be stored.

A PCM/ECM relay supply battery voltage, and/or ground signals are sent to the ECM, which need a battery voltage and ground signal, as well as the ignition switch to be in the “on” position to operate. The output signal also supplies the ECM with vehicle data. When the ECM is sent a signal that is outside of present manufacturer parameters, a code P0688 code can be stored.

What causes the P0688 code?

Several problems may cause a P0688 code including:

What are the symptoms of the P0688 code?

Symptoms of a P0688 code may include:

How does a mechanic diagnose the P0688 code?

A mechanic may use several methods to diagnose a P0688 trouble code, including:

  • Utilize an OBD-II scanner to check for a stored P0688 trouble code
  • Check for loose or corroded battery cables and cable ends
  • Check that the battery is fully charged
  • Perform battery load test
  • Perform a battery starting/charging system test
  • Check for damaged fuses
  • Test system fuses
  • Visually inspect the wiring and related electrical connectors for signs of damage or corrosion
  • Unplug ECM/PCM relay connector
  • Test related circuits for voltage and ground signals

Common mistakes when diagnosing the P0688 code

As replacing a vehicle’s ECM/PCM requires reprogramming, it is recommended to be sure to test and repair all other possible symptoms and problems that could cause a stored P0688 code.

How serious is the P0688 code?

Because a vehicle’s safe driving operation most likely will be impacted due to faults that store a P0688 code, this code is considered a potentially serious code. When this code appears, it is recommended to immediately take your vehicle to your local service center or mechanic for repair and diagnosis.

What repairs can fix the P0688 code?

Several repairs can fix a P0688 trouble code and include:

Need help with a P0688 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

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
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...
The Traveler’s Guide to Driving in Malaysia
CraigBurrows / Malaysia is a popular destination for many tourists today. The country has amazing sights and attractions that you will want to explore....

Related questions

Q: Total electric failure - 2003 Plymouth Aztec

The issue could be the battery. If it was connected wrong it may have worn it down and caused it to fail prematurely. If it was used, then there is even more reason to suspect that is the issue. Have...

Q: 2012 chrysler 200 won't start after replacing alternator which was bad after testing.

Hi there. It sounds like something may have shorted during the alternator replacement. If the positive cable going to the alternator made contact with anything metal, then it may have blown the main fuse. If not, then the battery should...

Q: How do you get Check Engine Light reading with no OBD-II plug?

All vehicles that are newer than 1996 and sold in the United States are required to have an OBD-2 system. The scanner plug may have been removed for some reason by the previous owner or mechanic. If this is the...