top of page

P0688 Fault Code (Causes & Fixes) ECM/PCM Power Relay Sense Circuit/Open

Updated: Feb 4

✔ This article has been fact checked.

A P0688 fault code means the engine control module (ECM), also known as a powertrain control module (PCM), indicated an excessive amount of power to the power relay sensing circuit, which powers the ECM/PCM.

This code is a generic powertrain fault code that refers to output circuit problems such as internal computer malfunctions.

P0688 is a sign of a potentially serious problem and may cause a breakdown, avoid driving the vehicle if this code appears and fix as soon as possible.

If a DTC is associated to the powertrain, an OBD scanner will read it as starting with P, thus the letter "P". Generic fault codes are those of the format P0xxx.

Table of Contents:

The Meaning of a P0688 Fault Code

When the relay control circuit that supplies power to the ECM sends out a high or abnormal voltage measurement, the trouble code P0688 is produced.

Vehicles that employ a PCM / ECM relay to deliver power and ground signals to the ECM will be the only ones in which this code will be generated.

A PCM / ECM relay sends battery voltage and ground signals to the ECM, which requires both battery voltage and ground signals, as well as the ignition switch to be in the "on" position, to function.

The output signal also provides vehicle data to the ECM. When the ECM receives a signal that is outside of the current manufacturer's specifications, this fault code is triggered.

Is It Serious?