P0688: ECM/PCM Power Relay Sense Circuit

What it means

The ECM or PCM has detected a fault in the power relay sense circuit. The code does not specify whether the signal is high or low.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The ECM or PCM power relay supplies power to the engine controller, while a sense circuit lets the controller check the relay’s output. P0688 sets when that sense circuit has a fault; the code does not say whether the signal is too high or too low.

Common causes

Most likely first.

1. Faulty power relay or contacts
2. Open or short in sense wiring
3. Poor connector contact

Symptoms

• Check engine light on
• Possible limp mode or no-start
• Multiple follow-up faults across modules
• Reset/relearn may be needed after repair

How to diagnose it

1. Record freeze-frame data and scan for related relay, voltage, or communication codes to see whether the fault occurs during starting or shutdown.
2. Check for intermittent controller resets, no-starts, or other module faults that may point to unstable relay output.
3. Inspect the relay, socket, and sense-wire connectors for corrosion, loose terminals, or damaged wiring.
4. Check the sense circuit for opens and shorts, then compare its signal with relay output using the manufacturer’s diagram and specifications.
5. Test relay contacts and confirm stable controller power and ground before considering a controller fault.

Common fixes

• Repair an open or short in the relay sense wiring.
• Clean or repair poor connections at the relay and sense-circuit connectors.
• Replace a relay with faulty contacts.
• Repair or replace the controller only after the relay output and sense circuit pass testing.

Can I keep driving?

An unreliable relay sense signal can accompany loss of controller power, causing a no-start or stall. Avoid driving if the engine cuts out or warning lights multiply, and arrange prompt diagnosis.