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

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:

• Damaged or defective ECM relay
• Damaged, defective, corroded or loose electrical wiring or related connectors
• Damaged or defective ECM/PCM

What are the symptoms of the P0688 code?

Symptoms of a P0688 code may include:

• Check engine or warning light may illuminate
• The check engine light may not illuminate even though there are stored codes that would illuminate it
• There may be no symptoms other than a stored code
• The vehicle may not start

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:

• Repair any loose or damaged battery cables
• Fully charge the battery
• Replace battery if needed
• Replace damaged or blown fuses
• Repair or replace damaged or corroded electrical wires and related connectors
• Repair loose or shorted connections
• Replace damaged or defective ECM/PCM relay
• Replace damaged or defective ECM
• Clear all codes, test-drive the vehicle, and rescan to see if any codes reappear

Need help with a P0688 code?

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