How is fault code P0685 fixed?

A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training. 1. Pull the codes and look at what else is stored alongside it, then check the live data while you crank. A P0606 or P0600 sitting with it points more at the ECU side than the relay 2. Get the relay out and look at it, then swap in a known-good one of the same rating. A spare from a non-critical circuit is often the same part number and proves it in two minutes 3. Check the relevant fuse and the relay socket for burnt or spread terminals, a relay that's been arcing leaves tell-tale browning 4. Back-probe the ECU power feed pin with the relay energised and look for a solid 12V or so. A reading that sags or drops in and out is your fault 5. Wiggle-test the loom and connectors while watching the voltage, intermittent opens nearly always show themselves when you move the wires by hand 6. If the relay, fuse and wiring all check out and you've still got an open on the control side, the ECU's internal relay driver is the remaining suspect

ECM/PCM Power Relay Control Circuit/Open

Jump to the fix

Severity

Medium

Can I keep driving?

Usually drivable, but get it diagnosed without delay.

First step

A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training. 1. Pull the codes and look at what else is stored alongside it, then check the live data while you crank. A P0606 or P0600 sitting with it points more at the ECU side than the relay 2. Get the relay out and look at it, then swap in a known-good one of the same rating.

PPowertrain
0Generic code
6Computer Output
85Sequence number

What does it mean?

OBD II fault code P0685 is a generic code that is defined as “ECM/PCM power relay -circuit open”, and is set when the PCM (Powertrain Control Module) detects that its own power supply has failed, or if it detects a low voltage in the circuit that supplies it with power. Note that this code only applies to applications that use a relay to supply battery power to the PCM. Most PCM power-supply circuits are relatively simple and straight forward.

When the ignition is turned to the “ON” position, power is fed to the relay, which causes an electromagnet to close two contact points- one of which is permanently connected to battery power. When this happens, a circuit that carries battery power is completed, and battery power is then fed to the PCM via a fused circuit. The image below shows the terminal configuration of a typical five-pin relay, such as might be found in the PCM power supply of many modern applications.

Symptoms

Likely causes

How to fix it

Codes From the Same System

All Fault Codes

Related Similar Codes

This information follows the generic OBD-II definition and does not replace a vehicle-specific diagnosis. The severity rating is rule-based; an authorised diagnostic reading is required for a definitive answer.