P0688

ECM/PCM Power Relay Sense Circuit/Open

Car Fault Codes Engine computer & electrical

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 15 minutes – 4 hours

Typical cost SAR 50 to 5,000

What it means

The engine computer (ECU) monitors the voltage coming out of the main relay through a sense circuit to confirm power actually arrived. The computer found that this voltage is not reaching it or the circuit is open, which can mean the engine parts fed by the relay are not getting power.

Symptoms4

No start or hard starting.
Sudden stalling or hesitation while driving.
Check engine light on, sometimes with codes for several sensors or injectors.
Reduced performance.

Possible causes5

Blown fuse between the relay and the computer or the parts it feeds.
Burnt contacts inside the main relay that no longer pass voltage.
Broken or corroded sense wire or connectors.
Weak battery or battery terminals.
Faulty sense circuit inside the engine computer (rare).

How to diagnose5

Check the battery and its terminals.
Check every fuse fed by the main relay according to the manufacturer wiring diagram.
Measure voltage at the relay output with the key on, then at the sense terminal at the computer.
Swap in an identical known-good relay to test.
Check continuity of the sense wire and connectors between the relay and the computer.

How to fix5

Replace a blown fuse and find the reason it blew.
Replace the main relay if its contacts are burnt.
Repair the broken sense wire or corroded connector.
Clean the battery terminals or replace a weak battery.
Clear the codes and confirm normal starting and that the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The fix is usually a fuse, relay or wire repair at low cost; the upper end applies only if the computer is proven faulty.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

What is the difference between the control and sense circuits?

The control circuit is how the computer switches the relay on, and the sense circuit is how it checks that voltage actually left the relay. This code means the command was sent but the voltage was not seen.

Why did injector or sensor codes appear with it?

On many vehicles the main relay feeds them, so when it does not pass voltage they lose power and the computer logs codes for them.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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