Fix P0685: Mechanic’s solutions that WORKED

The P0685: ECM/PCM Power Relay Control Circuit/Open indicates a malfunction in the control circuit of the ECM/PCM power relay, which is a critical component for the operation of the vehicle’s main computer. This relay helps in managing the power supply to the ECM/PCM, and any issues here can impact the overall performance and starting capabilities of the vehicle.

P0685 quick overview

Meaning	P0685: ECM/PCM Power Relay Control Circuit/Open
Is it serious?	This code can be serious as it relates to the main computer (ECM/PCM) of the vehicle, potentially leading to starting issues or erratic vehicle behavior.
Possible causes	– Faulty ECM/PCM power relay – Open or short in the ECM/PCM power relay control circuit – Poor electrical connections in ECM/PCM power relay circuit – Faulty ECM/PCM
How to diagnose?	Check ECM/PCM Relay: Inspect the relay for damage, proper operation, and secure connections. Inspect Wiring and Connectors: Examine the wiring and connectors in the ECM/PCM circuit for opens, shorts, or poor connections. Test Electrical Connections: Perform electrical tests on the ECM/PCM power relay circuit to identify any anomalies. Replace ECM/PCM Relay: If the relay is found to be faulty, replace it with an appropriate part. Check for Aftermarket Influences: Look for any aftermarket parts that could be influencing the ECM/PCM operation. Reset and Clear Codes: After repairs, reset the ECM/PCM and clear diagnostic codes, then monitor for recurrence.

How to fix P0685 – learn from mechanics

I looked into P0685 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Honda

Vehicle Information: 2003 Honda Accord EX 3.0L – Stall, MIL Lamp on

Issue + Repair History:

• Accord stalls at idle occasionally.
• Problem occurred a month ago, then resolved, and recurred yesterday.
• Check engine light activated.
• Instrument cluster and radio experienced intermittent failure.
• ECU relay removal had no effect on radio or instrument cluster issues.
• Car running fine currently.

Other OBD2 Codes + Test Results:

• P0685 – ECU power control circuit malfunction.
• Battery check: OK.
• Alternator check: OK.
• Engine grounds: OK.
• Battery inspection and cleaning: OK.

Suggestions to Fix Issue:

• Inspect ignition switch and its wiring harness.
• Check ground connection under the thermostat.
• Verify power supply integrity to underhood fuse box.
• Investigate main PGM FI relay, especially if kickpanel is damaged.
• Update PCM software or check for loose wires/connectors to PCM.
• Consider replacing PCM if other steps fail.
• Examine main relay, particularly in older Honda models.

Final Fix:

• Issue resolved by addressing high voltage drop at ground connection at thermostat housing.
• Ground connection cleaned and tightened.
• Symptoms not duplicated in shop, but customer to monitor car performance.

Case #2: Ford

Vehicle Information: 2008 Ford Edge SE 3.5L – Surge, Cuts out, Stall

Issue + Repair History:

• Vehicle involved in a front-end collision in May; driven since then without apparent issue.
• Brought in for intermittent stalling; issue not initially replicable during tests.
• P0230 code consistently resetting.
• New fuel pump installed; stalling resumed after a few days.
• Vehicle now stalls during extended test drives.
• Additional codes: P0685 (PCM relay circuit), P1260 (theft deterrent).
• Replaced weak crankshaft sensor and throttle body, but stalling persisted.
• APP sensors tested good.

Computer Codes & Descriptions:

• P0230: Fuel pump circuit.
• P0685: PCM power relay circuit.
• P1260: Theft PATS system.

Suggestions to Fix Issue:

• Check grounds and for any chafed wires.
• Investigate wiring for the PCM power relay; check for issues due to collision repair.
• Replace PCM and Fuel Pump relays and fuses.
• Monitor fuel pressure; test fuel relay for PATS system involvement.
• Consider harness or PCM issues as indicated by diagnostic tests.

Final Fix:

• Issue caused by incompatible aftermarket cooling fans installed post-accident.
• Fans likely had a feedback problem or were wrong for the vehicle.
• Replacing with OEM fans resolved the issue.

Case #3: Dodge

Vehicle Information: 2005 Dodge Grand Caravan SE Plus 3.8L – No Start

Issue + Repair History:

• Caravan won’t start unless ASD relay circuit 30 is jumped.
• No ground activation from the computer at circuit 85 of the relay.
• Powers present at relay circuits 86 and 87.
• Codes PO685 and PO688 appear; PO685 reappears immediately after clearing, without key cycling.
• Relay slot shows flashing ground with 0.3 resistance using a power probe.
• Uncertain if issue is with powertrain control module, integrated power module, or fuse relay center.

Computer Codes & Descriptions:

• PO685: ASD control circuit relay.
• PO688: ASD sense circuit low.

Suggestions to Fix Issue:

• Inspect under PDC for a green wire or near the brake master.
• Check for the ‘green weenie’ issue known in these models; look for corroded or broken pins on PDC board.
• Chrysler provides a plug-and-play replacement box for these cases.
• Examine wires between PCM and ASD for corrosion or damage; perform a continuity test.

Final Fix:

• Discovered a chewed green wire and a broken brown wire after removing and inspecting the fuse relay box.
• Repairing these wires resolved the issue, and the car is operational again.

Case #4: Chrysler

Vehicle Information: 2004 Chrysler Pacifica 3.5L – No Start, Stall

Issue + Repair History:

• Vehicle returned with no cranking, no start, and a “P0513 service immobilizer” message.
• No communication with PCM, but able to communicate with BCM and SKIM.
• Tested power, grounds, data links, and harness; no issues found.
• PCM initially sent back for review; reported as locked in theft mode, disabling injectors.
• After PCM return, still had P0513 code, plus additional codes indicating PCM and relay failures.
• Temperature sensor and other readings erratic; PCM communication lost intermittently.
• Initial PCM replacement was due to intermittent stall and no start; no issues before PCM replacement.

Computer Codes & Descriptions:

• SKIM: PCM failure, serial link external failure.
• P0685: ASD relay circuit.
• P0627: Fuel pump relay.
• P0645: A/C clutch relay.
• P0513: Service immobilizer.

Suggestions to Fix Issue:

• Recheck all grounds, particularly G103 & G104 on the frame front.
• Examine 5-volt reference voltage; check for faulty sensors affecting PCM.
• Inspect main ground at transmission; load test all powers and grounds at PCM.
• Consider resetting all computers to facilitate communication.
• Use dealer PCM as aftermarket ones may not be reliable.

Final Fix:

• Issue due to a bad aftermarket computer.
• Multiple replacements from rebuilder caused different problems.
• Replacing with a dealer PCM resolved the issue.

Case #5: Ford

Vehicle Information: 2008 Ford Escape Hybrid 2.3L

Issue + Repair History:

• Assisted in unplugging the hybrid system at a local body shop due to right rear impact.
• Following day, after reconnecting everything, the car wouldn’t start.
• Codes P0685, P01A10, P1A14, B2172 appeared.
• No blown fuses or bad relays detected.
• Questioned if there’s a relearn or flash procedure after battery disconnection.

Computer Codes & Descriptions:

• P0685, P01A10, P1A14, B2172 (specific descriptions not provided).

Suggestions to Fix Issue:

• Check inertia switches; vehicle has two, front and rear, which disable fuel and high voltage supply when open.
• B2172 code directly related to inertia switches.
• Clear BCM and PCM separately; try resetting by removing rear fuse, reinstalling, then removing front battery negative cable.
• Retest PCM, clear all codes, then test BCM and clear codes.
• Check PCM relay and Inertia Fuel Shutoff (IFS) switch; ensure proper safety precautions.

Final Fix:

• Problem traced to the back inertia switch, likely due to rear-end collision.
• Disconnected battery and turned off hybrid battery, then reinstalled and let car sit for 20 minutes.
• This procedure allowed the PC to reload, resolving the issue.

Advice	Brief Description of Diagnostic Step
Check PCM Relay and Connections	Inspect the Powertrain Control Module (PCM) relay for proper function and secure connections.
Inspect Grounds and Wiring	Examine all relevant grounds and wiring, particularly around areas impacted by collisions or repairs.
Clear and Reset PCM and Other Modules	Clear codes from the PCM and, if necessary, other modules, then perform a reset procedure.
Investigate Inertia Switches	Check inertia fuel shutoff switches, especially in cases of vehicle impact or collision.
Consider Quality of Replacement Parts	Use OEM parts for replacements, especially for critical components like PCMs and relays.
Perform Thorough Diagnostic Tests	Conduct comprehensive diagnostic tests including checking for power, grounds, and data links.
Look for External Influences and Feedback	Search for external factors like aftermarket parts causing feedback or compatibility issues.