ECM / PCM · RELAY CONTROL · NO START
An ECU power-relay code identifies a monitored electrical condition. It does not tell you to replace the engine controller without further testing. The relay arrangement, supply path and monitoring method vary between vehicles.
Control circuit and feedback circuit
P0685, P0686 and P0687 concern the ECM/PCM power-relay control circuit: a circuit/open condition, low level or high level respectively. P0688, P0689 and P0690 concern the relay’s monitored feedback: a circuit condition, low level or high level respectively.
These short descriptions are a starting point. Use the full scan wording and vehicle-specific circuit diagram. Reference: HELLA powertrain code reference.
Record the starting behaviour
Does the engine crank normally without starting, fail to crank, start then stop, or cut out when warm? Does the scan tool communicate with the engine controller at the moment of failure? Note whether a key cycle temporarily restores operation and whether other controllers report losing contact with the engine ECU.
What the workshop needs to establish
- Correct supply and earth at the affected circuit under the relevant operating conditions.
- Condition of the fuse, relay, connector and wiring, using the correct diagram.
- Whether the relay command and monitored result agree.
- Any moisture, heat damage or previous wiring modifications.
This is an enquiry checklist, not a pinout. Do not bridge relay terminals using a generic internet diagram: the circuit must first be identified for the vehicle.
Can the ECU still be involved?
Yes, a controller or its output stage may require assessment once the surrounding circuit has been investigated. Send the test results rather than only a screenshot of the code. If the vehicle-side fault is still unknown, our complex electronics diagnostic service is a useful starting point. If your garage has isolated a module issue, discuss ECU bench assessment before dispatch.
Send AXION your diagnostic details
For a Didcot workshop enquiry or a suitable UK postal module assessment, send the vehicle model, year, engine or gearbox, full module label, complete scan report and a description of what happened. Include previous battery work, programming, water ingress or replacement parts. Please agree suitability before removing or posting a unit.