Faulty or failing ECM/PCM power relay (sticking or worn contacts, weak coil); Wiring fault on the relay control circuit, such as a short to earth or chafed insulation; Corroded, loose or damaged connectors or terminals at the relay, fuse box or control module; Poor battery condition, weak charging or low supply voltage affecting the relay circuit; Blown or poor-contact fuse in the relay supply or control circuit; Faulty ignition switch or body control module supplying the relay circuit (model dependent); Internal fault in the ECM/PCM driver circuit.

P0686

ECM/PCM Power Relay Control Circuit Low.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
86: FaultThe specific fault in that area

What it means

P0686 means the engine or powertrain control module has detected a low voltage in the control circuit of its main power relay. The module commands the relay on and off, and it sets this code when the voltage on the relay control circuit does not match what it expects, for example when it is pulled low or the relay fails to switch as commanded. Relay location, circuit layout and the exact setting conditions vary by model and engine.

Symptoms

Engine cranks but does not start, or starts then stalls
Intermittent no-start or hard starting
Engine cutting out while driving
Malfunction indicator lamp on, possibly with other warning lights
Other control module communication faults logged at the same time
Loss of power or limp-mode behaviour

Possible causes

Faulty or failing ECM/PCM power relay (sticking or worn contacts, weak coil)likely
Wiring fault on the relay control circuit, such as a short to earth or chafed insulationlikely
Corroded, loose or damaged connectors or terminals at the relay, fuse box or control modulelikely
Poor battery condition, weak charging or low supply voltage affecting the relay circuitpossible
Blown or poor-contact fuse in the relay supply or control circuitpossible
Faulty ignition switch or body control module supplying the relay circuit (model dependent)rare
Internal fault in the ECM/PCM driver circuitrare

How to diagnose

Read all stored and pending codes and freeze frame data, noting battery voltage and engine state when the code set, then check for other power or communication codes
Test battery condition and charging voltage, and check the battery terminals and earth connections for tightness and corrosion
Visually inspect the relay, fuses and fuse box for heat damage, water ingress, corrosion and bent terminals
Check the relevant fuses for continuity and voltage drop under load, using the wiring diagram for the specific model
Swap the relay with a known good identical one, or test the relay coil resistance and contact operation on the bench
With the relay removed, check the control circuit for voltage, short to earth and continuity back to the control module, following the wiring diagram
Wiggle-test the harness and connectors while monitoring live data or the circuit to find intermittent faults
If everything else checks out, verify the module's own power and earth supplies before suspecting the module itself

How to fix

Replace the ECM/PCM power relay with the correct specification part
Repair or replace damaged wiring on the relay control circuit, protecting it from further chafing
Clean, repair or replace corroded or damaged connectors and terminals
Recharge or replace a weak battery and repair any charging system fault
Replace blown fuses only after finding and fixing the cause of the overload
Repair or replace the control module only after all supplies, earths and wiring are proven good, and follow the manufacturer's procedure for programming

Can I keep driving?

High severity

Avoid driving until this is diagnosed, as the engine may cut out without warning, which is a safety risk, and a no-start is likely. If the car is running normally and the fault is intermittent, limit driving to getting it to a workshop.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.