P0690

What it means

P0690 means the engine or powertrain control module has detected a voltage that is too high on the sense circuit that monitors the main power relay's output. The control unit compares the voltage it sees on this circuit with what it expects when the relay is switched on or off, and sets the code when the reading is higher than it should be. How the relay is wired and when the check runs varies between models and engines.

Symptoms

• Engine cranks but will not start, or starts and then stalls
• Intermittent loss of power or engine cutting out while driving
• Malfunction indicator lamp on
• Hard starting or long cranking time
• Other fault codes stored, especially for sensors or actuators that share the relay supply
• Engine management warning or limp mode on some models

Possible causes

• Main power relay contacts stuck closed or relay failing internally, so the sense circuit stays live when the relay should be offlikely
• Corroded, loose or damaged connectors or terminals at the relay, fuse box or control unitlikely
• Short to voltage on the relay sense wire, caused by chafed or damaged wiringpossible
• Charging system fault causing overvoltage, or a poor battery connection giving unstable system voltagepossible
• Previous repair or aftermarket electrical work that has altered or back-fed the relay circuitpossible
• Internal fault in the engine or powertrain control modulerare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting battery voltage and engine state when the code set, and check for related power supply codes
2. Check the battery condition and terminals, then measure system voltage with the engine off and running to rule out overvoltage or unstable supply
3. Inspect the relay, fuse box and control unit connectors for water ingress, corrosion, heat damage or bent and pushed-back pins
4. Inspect the wiring harness for chafing, rubbing or previous repairs along the relay sense circuit, using the wiring diagram for the specific model
5. Test the power relay: check that it clicks, measure coil resistance and check the contacts open and close properly, or swap it with a known good identical relay if the manufacturer permits
6. With the relay off or removed, check the sense circuit for unwanted voltage, and check the wiring for a short to supply or to another circuit
7. Check the control unit supply and earth circuits, and only consider the control unit itself once all wiring and the relay have been proven good

How to fix

• Replace the main power relay if it is sticking or testing faulty, using the correct specification
• Clean, repair or replace corroded or damaged connectors and terminals
• Repair or replace damaged wiring on the sense circuit, removing any short to voltage
• Repair the charging system or battery connections if system voltage is too high or unstable
• Correct any faulty aftermarket or previous electrical modifications that affect the relay circuit
• Repair or replace the control module and have it programmed as the manufacturer requires, only if it is confirmed faulty

Can I keep driving?

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