Faulty or worn main power relay (contacts or coil); Corroded, loose or damaged relay socket terminals or fuse box contacts; Open circuit, chafed wire or high resistance in the relay control or supply wiring; Blown fuse feeding the relay or control module supply; Poor battery connection, weak battery or poor earth connections; Ignition switch or related circuit fault interrupting the relay signal; Internal fault in the control module relay driver.

P0685

ECM/PCM Power Relay Control Circuit/Open.

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
85: FaultThe specific fault in that area

What it means

P0685 means the engine or powertrain control module has detected an open circuit or no response in the control circuit for the main power relay. The module commands the relay to switch on and monitors the circuit and the resulting supply voltage. If the relay does not energise or the expected voltage is missing, the code is set. Relay layout and monitoring vary by manufacturer and model.

Symptoms

Engine cranks but will not start, or fails to start intermittently
Engine cuts out while driving or stalls without warning
Malfunction indicator lamp on
Loss of communication with the engine control module
Other warning lamps or erratic gauge behaviour
Engine runs but goes into limp mode or has poor performance

Possible causes

Faulty or worn main power relay (contacts or coil)likely
Corroded, loose or damaged relay socket terminals or fuse box contactslikely
Open circuit, chafed wire or high resistance in the relay control or supply wiringlikely
Blown fuse feeding the relay or control module supplypossible
Poor battery connection, weak battery or poor earth connectionspossible
Ignition switch or related circuit fault interrupting the relay signalpossible
Internal fault in the control module relay driverrare

How to diagnose

Read all stored and pending codes and note freeze frame data, including battery voltage and engine state when the code set
Check the battery state of charge and terminal condition, and confirm battery and earth cable connections are clean and tight
Check the fuses related to the engine control module and main relay using the vehicle wiring diagram, as locations vary by model
Inspect the relay, its socket and the fuse box for heat damage, corrosion, green deposits or bent terminals
Listen and feel for the relay clicking when the ignition is switched on, then swap with a known good identical relay if the manufacturer permits
With a multimeter, check supply voltage, control signal and earth at the relay socket against the wiring diagram
Test the relay control wire between the relay and control module for continuity, short circuits and resistance, wiggling the harness while monitoring
Check the module power and earth feeds at the connector before suspecting the module itself

How to fix

Replace the faulty power relay with the correct specification part
Clean or repair corroded relay socket terminals, or replace the socket or fuse box section if damaged
Repair or replace damaged wiring and connectors in the relay control or supply circuit
Replace blown fuses only after finding and fixing the cause of the overload
Repair or replace poor battery terminals, cables or earth connections, and replace a failing battery
Repair or replace the control module only after all power, earth and wiring checks have been confirmed good, following the manufacturer procedure

Can I keep driving?

High severity

Avoid driving until it is diagnosed, as the engine may stall without warning or fail to restart, which is a safety risk in traffic. If the fault is clearly intermittent, only make short, necessary journeys and get it checked promptly.

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