P0685: ECM/PCM power relay control circuit open

Engine computer power relay control circuit

Severity: Inspect soon

P0685 means the engine computer sees a fault in the circuit it uses to switch its own power relay. If that relay drops out, the engine computer and the parts it feeds lose power, which can cause stalling or a no-start. Get it inspected soon and stop driving if it stalls.

It gets more serious if:

Stop: The engine stalls, shuts off at stops or while driving, or will not start. Owners with P0685 report stalls at city speeds and at traffic lights, and REREV advises against driving with this code. Do not keep driving; have it towed or checked where it sits.

Source: how we rate severity

P0685 in short

P0685 means the engine computer found a fault in the control circuit for its power relay, the relay that feeds the engine management system. Common causes are the relay, a fuse or wiring fault, and low battery voltage. The engine may stall or not start. Get it inspected soon, and stop driving if it stalls.

Many engines power the engine computer, injectors, coils and sensors through a relay that the computer itself switches on. GM calls it the powertrain relay; other makers call it the ECM, PCM or main relay.

P0685 sets when the computer's relay driver does not behave as commanded. In GM's logic, the commanded and actual driver states must disagree for more than 2 seconds with the ignition on and battery voltage between 11 and 32 volts.

The relay, its fuse, the wiring and a weak battery are the usual suspects. On some vehicles the relay sits inside a fuse box or integrated power module, which changes what gets replaced.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

Cause 1: Failed engine computer power relay
Common general cause
Why it ranks here
The relay is the part being controlled. GM's procedure ends with replacing the relay when the circuits test normal, and REREV names it as the most common cause.
What would change this
Rises if the relay does not click when commanded or its coil resistance is out of range; falls if a known-good relay changes nothing.
How to test the engine controls ignition relay
Cause 2: Blown fuse, damaged wiring or poor connection in the relay circuit
Common general cause
Why it ranks here
The relay needs battery feed, ignition feed and a clean control circuit; a fuse, broken wire or loose connector stops it working. A dealer replaced the powertrain relay fuse on a 2015 CR-V with P0685.
What would change this
Rises if a fuse is blown or a test lamp shows a missing feed at the relay terminals.
How to test the underhood fuse block (engine compartment fuse box)
Cause 3: Weak battery or charging system fault
Possible — needs testing
Why it ranks here
Low voltage can upset relay operation and the computer's monitoring. A dealer replaced the battery along with other parts on a 2015 CR-V with P0685.
What would change this
Rises if the battery fails a load test or charging voltage is low.
Cause 4: Engine computer fault or software
Possible — needs testing
Why it ranks here
A failed relay driver in the computer or a software issue is possible but less common; owners report dealers updating software or replacing the computer for P0685 on some Hondas.
What would change this
Only rises once the relay, fuses, wiring and battery test good, or a dealer update applies.
How to test the engine control module (ecm / pcm)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

Step 1: Have the battery and charging system tested.
Low riskTool: battery tester or multimeter
- IfThe battery fails or charging voltage is low
  It meansFix the battery or charging fault, clear the code and retest.
  Points to: Weak battery or charging system fault
- IfBattery and charging are good
  It meansCheck the fuse and relay.
  Go to step 2
Step 2: With the key off, find the engine computer power relay and its fuses in the owner's manual or service information, check the fuses and swap the relay with an identical one if the fuse box has one.
Low riskTool: owner's manual, fuse tester
- IfA fuse is blown or a terminal is loose or burnt
  It meansRepair the cause and replace the fuse.
  Points to: Blown fuse, damaged wiring or poor connection in the relay circuit
- IfThe code clears with the swapped relay
  It meansThe relay has failed.
  Points to: Failed engine computer power relay
- IfNo change
  It meansA shop should test the circuit.
  Go to step 3
Step 3: Have a shop command the relay with a scan tool, test the feeds at the relay terminals with a test lamp and measure the relay coil resistance.
Best left to a shopTool: scan tool with output controls, test lamp, multimeter, wiring diagram
Warning:Probing powered relay circuits can blow fuses or damage the engine computer if the wrong terminals are bridged.
- IfFeed or control circuit open, shorted or high resistance
  It meansRepair the wiring.
  Points to: Blown fuse, damaged wiring or poor connection in the relay circuit
- IfRelay coil resistance out of range (GM lists about 70-110 or 200-250 ohms)
  It meansReplace the relay.
  Points to: Failed engine computer power relay
- IfAll circuits and the relay test good
  It meansCheck for software updates or an engine computer fault. On some Chrysler models the relay is inside the integrated power module; FCA asks technicians to call before replacing it for P0685.
  Points to: Engine computer fault or software

Symptoms

Check engine light on, sometimes on and off
Hard starting
Engine shuts off at stops or while driving Gets more serious: stop and have it checked.
Will not start Gets more serious: stop and have it checked.

How to fix

Step 1: Write down every stored code and the freeze-frame data before clearing anything.
Step 2: Have the battery tested first; a weak battery is a cheap thing to rule out.

Parts and cost context

Engine controls ignition relay:Relays are cheap, but confirm the fuses, feeds and battery are good first; on some vehicles the relay is built into a fuse box or power module.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Honda

2007 · Accord
Supported by vehicle data
Owners report the engine shutting off at stops with P0685, and dealers applying engine computer software updates. Have the relay, fuses and battery checked as well.
2015 · CR-V
Supported by vehicle data
An owner reports stalls at city speeds with P0685; the dealer replaced the battery, engine computer and powertrain relay fuse.

All Engine computer (ECM/PCM) codes

Sources and review

DTC P0685, P0689, P0690 or P1682, 2011 Saab 9-4X service information (P0685 section) — Saab / General Motors service information (saabwisonline.com)
P0685: ECM/PCM Power Relay Control Circuit Open — REREV
NHTSA complaint ODI 11768416: 2015 Honda CR-V, stalls at city speeds, dealer confirmed P0685 — NHTSA Office of Defects Investigation (complaint flat file) , September 18, 2026
NHTSA complaint ODI 11375673: 2007 Honda Accord, engine shuts off at stops, P0685 — NHTSA Office of Defects Investigation (complaint flat file) , November 19, 2020
NHTSA complaint ODI 10505068: 2007 Honda Accord, intermittent electrical faults, dealer found P0685 and updated the PCM — NHTSA Office of Defects Investigation (complaint flat file) , February 25, 2013
How To Test An Automotive Relay At Home — Mechanic Base (Magnus Sellén, automotive diagnostic technician)
Chrysler TIPM tech line note: call before replacing a TIPM for no start with P0685 or P0688 — FCA US LLC (Chrysler) / NHTSA , December 2, 2015
DTC P0685, P0689, P0690 or P1682 (LFW), 2011 Saab 9-4X service information — Saab Automobile / General Motors (Saab WIS online archive)

Last reviewed: October 15, 2026
Written by: Saurabh K.
Editorial review: Pending
Technician review: Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

Severity: Inspect soon

Most likely causes

Cause 1: Failed engine computer power relay

Cause 2: Blown fuse, damaged wiring or poor connection in the relay circuit

Cause 3: Weak battery or charging system fault

Cause 4: Engine computer fault or software

What to test first

Step 1: Have the battery and charging system tested.

Step 2: With the key off, find the engine computer power relay and its fuses in the owner's manual or service information, check the fuses and swap the relay with an identical one if the fuse box has one.

Step 3: Have a shop command the relay with a scan tool, test the feeds at the relay terminals with a test lamp and measure the relay coil resistance.