Fix P0688: Mechanic’s solutions that WORKED

The code P0688: ECM/PCM Power Relay Sense Circuit / Open indicates a problem in the electrical circuit of the ECM/PCM power relay. This relay plays a critical role in supplying power to the Engine Control Module (ECM) or Powertrain Control Module (PCM), which are central to the vehicle’s engine management system.

P0688 quick overview

Meaning	P0688: ECM/PCM Power Relay Sense Circuit / Open
Is it serious?	This code is serious as it can lead to vehicle stalling, failure to start, or other significant drivability issues.
Possible causes	– Faulty ECM/PCM power relay – Open or shorted ECM/PCM power relay harness – Poor electrical connection in the ECM/PCM power relay circuit – Faulty engine control module (ECM) or powertrain control module (PCM)
How to diagnose?	Inspect ECM/PCM Relay: Check the ECM/PCM power relay for any damage or faults. Check Wiring and Connectors: Examine wiring and connectors in the circuit for damage or loose connections. Test Relay Circuit: Perform electrical tests on the relay circuit to identify any shorts or opens. Evaluate ECM/PCM: Assess the condition of the ECM or PCM, considering replacement if faulty. Check Ground Connections: Ensure proper grounding of the ECM/PCM and related components. Monitor Voltage Levels: Test voltage levels at key points in the circuit to identify irregularities.

How to fix P0688 – learn from mechanics

I looked into P0688 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: Jeep

Vehicle Information:
2008 Jeep Grand Cherokee Limited 4.7L

Issue + Repair History:

• The vehicle cuts out after running for 10-15 minutes but restarts right away.
• Initially had codes P0688 and P0305.
• Replaced spark plugs and coil, checked wiring to relay, and replaced PCM.
• After these repairs, only code P0688 remained.

Suggestions to Fix Issue:

• Replace all spark plugs and coils, preferably with OEM parts.
• Check and clean all ground connections near the battery.
• Consider the possibility of faulty crank sensor or oil pressure sensor.
• Inspect for 5-volt reference loss, possibly caused by crank sensor or other sensors.
• Ensure the use of correct type and gap for spark plugs.
• Consider quality of PCM, as some remanufactured PCMs may be faulty.

Final Fix:

• Replaced all spark plugs with two different types for exhaust and intake sides as specified by Dodge.
• Replaced all coils with new parts from O’Reilly Auto Parts.

Case #2: Chrysler

Vehicle Information:
2005 Chrysler Pacifica 3.8L

Issue + Repair History:

• Experienced a no-crank condition with code P0688 (ASD relay sense low).
• Initially had issues with running badly, not idling, and low power.
• After driving home, it stopped running, and excessive cranking led to a no-crank problem.
• Despite testing, power and grounds at various points were found to be good.
• Manually grounding the relay could turn the engine over, but it wouldn’t start or ground the fuel pump or ASD relays.

Suggestions to Fix Issue:

• Suspected a faulty PCM, especially after excessive cranking by the customer.

Final Fix:

• Confirmed a bad PCM by installing a used one, after which the car could start and run.

Case #3: Dodge

Vehicle Information:
2005 Dodge Grand Caravan SE Plus 3.8L

Issue + Repair History:

• The vehicle would not start unless the ASD relay circuit number 30 was jumped.
• No ground activation from the computer at circuit 85 on the relay, but power was present at 86 and 87.
• Codes PO685 (ASD control circuit relay) and PO688 (ASD sense circuit low) were present.
• Issues with flashing ground and resistance at the relay slot.

Suggestions to Fix Issue:

• Check under PDC for issues with wiring, particularly a green wire, or near the brake master.
• Inspect for corroded or broken wires between PCM and ASD relay.
• Consider the possibility of a faulty PCM or issues in the integrated power module.

Final Fix:

• Discovered a chewed green wire and a broken brown wire after removing the fuse relay box and cutting back some tape. Repairing these wires fixed the issue.

Case #4: Volkswagen

Vehicle Information:
2002 Volkswagen Jetta GLS 1.8L

Issue + Repair History:

• The car stalls when ambient temperature is high, triggering the EPC light.
• Scanned codes include P0300 (random misfire), P0301 (misfire cyl 1), P0303 (misfire cyl 3), P0562 (power supply voltage too low), and P0688 (main relay J271 open circuit).
• Wiring and voltage to the coils and relay were checked and found to be fine.

Suggestions to Fix Issue:

• Replace the J271 relay, which is known to fail intermittently, especially in high temperatures.
• Consider updating all the coils along with the relay due to wear and tear.
• Inspect the solder joints on the relay for any cracks and resolder if necessary.
• Test the relay with a heat gun to simulate high temperatures and observe its behavior.

Final Fix:

• Replaced the J271 relay, resolving the stalling issue.

Case #5: Volkswagen

Vehicle Information:
2006 Volkswagen Passat 2.0T 2.0L

Issue + Repair History:

• Exhibited poor idle quality, hesitation, misfire, and MIL lamp on.
• Numerous DTCs including oxygen sensor, multiple cylinder misfires, and ECM power relay issues.
• Swapped coils, which resolved some misfires.
• Client added fuel and octane booster, improving vehicle performance.
• Vehicle had an engine replacement prior and never ran correctly afterwards.

Suggestions to Fix Issue:

• Check wiring harness from PCM to engine components for issues.
• Investigate blown fuses, bad sensors, or failing relays for the DME power supply.
• Consider battery health and ensure it’s in good condition.
• Look for water intrusion or corrosion in the Ebox, especially around relays.

Final Fix:

• Dealership found a recall for PCM replacement. Replacing and reprogramming the PCM resolved most of the issues. The high-pressure fuel pump code still remains.

Takeaway Advice for P0688	Description of Diagnostic Step
Inspect PCM and Wiring	Check the Powertrain Control Module and its wiring for faults.
Evaluate ASD Relay and Circuit	Examine the Auto Shutdown (ASD) relay and associated circuitry for issues.
Check Battery and Charging System	Ensure the battery is healthy and the charging system functions correctly.
Review Ground Connections	Verify proper grounding in relevant electrical circuits.
Assess Related Fuses and Relays	Check fuses and relays connected to the ECM and related systems.
Monitor for Voltage Irregularities	Look for any voltage issues that might indicate wiring or relay problems.
Consider Excessive Cranking Impact	Evaluate the effect of excessive engine cranking on the PCM and electrical system.
Investigate Wiring Harness Issues	Inspect the wiring harness for damage, corrosion, or poor connections.
Look for Signs of Water Intrusion	Check for water damage or corrosion, especially in fuse and relay boxes.