Fix P0651: Mechanic’s solutions that WORKED

The code P0651: 5-Volt Reference 2 Circuit indicates an issue with the ‘5-Volt Reference 2 Circuit’, which is essential for the proper functioning of various sensors in a vehicle. This code is triggered when the Engine Control Module (ECM) detects an abnormality in the circuit, typically due to issues like sensor shorts, wiring problems, or ECM faults.

P0651 quick overview

Meaning	P0651: 5-Volt Reference 2 Circuit
Is it serious?	This code can be serious as it may affect the performance of multiple sensors and systems in the vehicle.
Possible causes	– Shorted sensor on the 5-volt reference circuit – Faulty Engine Control Module (ECM) – Damaged or corroded wiring and connectors – Faults in the 5-volt reference circuit within the ECM – Poor ECM ground or power connections
How to diagnose?	Inspect Wiring: Check for damaged or corroded wiring and connectors. Check Sensors: Examine sensors on the 5-volt reference circuit for shorts. Test ECM Outputs: Verify if the ECM is properly outputting 5 volts. Disconnect Sensors Individually: Isolate the issue by disconnecting each sensor and testing. Load Test Circuits: Conduct load tests on circuits. Check ECM Grounds/Power: Ensure all ECM connections are secure.

How to fix P0651 – learn from mechanics

I looked into P0651 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: Chevrolet

Vehicle Information: 2003 Chevrolet Silverado 1500 WT 4.3L

Issue + Repair History:

• Intermittently sets codes P0651, P0641, and later P0116.
• Previously had P0651 and P0641 cleared.
• Experienced hard start, with reference voltage issues.

Other OBD2 Codes + Test Results:

• Codes P0651, P0641, and P0116 reoccurred.
• Freeze frame data indicated issues with coolant temperature and reference voltage.

Suggestions to Fix Issue:

• Check the ECM, including putting it in a freezer or applying heat.
• Examine all 5-volt sensors, especially the fuel tank pressure sensor.
• Inspect wiring harness for damage or chafing.
• Perform voltage drop tests on ECM power and ground circuits.
• Consider possible ECM fault if no security or U codes are present.

Final Fix:

• Replaced the ECM, resolving the issues. ECM needed programming and testing over several days.

Case #2: Chevrolet

Vehicle Information: 2008 Chevrolet Express 1500 4.3L

Issue + Repair History:

• Intermittent code P0641.
• No repairs or parts replaced initially.
• Problem occurs within a short driving distance.

Other OBD2 Codes + Test Results:

• p0641: 5v reference 1 circuit.

Suggestions to Fix Issue:

• Monitor all 5v reference sensors with a laptop to identify the fault during occurrence.
• Inspect wiring for shorts or damage.
• Check all connections and grounds for issues.

Final Fix:

• Loose battery cables were identified and fixed. New cable bolts with auxiliary attachments were installed, resolving the issue.

Case #3: Pontiac

Vehicle Information: 2004 Pontiac Grand Prix GTP 3.8L

Issue + Repair History:

• Hard shifting, MIL on.
• Codes P0651 and P0742 were present.
• Checked for a short to ground on the TCC release switch without success.
• Initially suspected PCM issues.
• Vehicle developed a no-start condition with no injector pulse after ECM was reinstalled.

Other OBD2 Codes + Test Results:

• P0651: 5v reference 2 circuit.
• P0742: TCC system stuck on.

Suggestions to Fix Issue:

• Check the transmission fluid pressure switch.
• Monitor TCC pressure switch status with a scanner.
• Explore potential valve body or TCC/PWM problems in the transmission.

Final Fix:

• Replaced the PCM with a salvaged one and had it programmed. This resolved the issues, indicating the original PCM was faulty.

Case #4: GMC

Vehicle Information: 2006 GMC Sierra 1500 SLE 5.3L

Issue + Repair History:

• AC system intermittently failing with compressor shutting off and blower speed irregularities.
• No power to compressor, low and high-pressure switches; relay receiving power but no ground from PCM.
• Replaced AC control head/panel and relay; AC system recharged.
• Code P0651 present.

Suggestions to Fix Issue:

• Check the 5V reference line for proper voltage.
• Inspect wiring, especially around the PCM and the high-pressure switch for shorts or damage.
• Command AC on with a scanner to check if PCM is shutting down the compressor due to system issues.
• Consider examining the harness under the driver side carpet for potential wiring issues.

Final Fix:

• A short in the wire for the high-pressure switch near the PCM was repaired.
• Blower issue resolved by replacing the resistor.

Case #5: Volkswagen

Vehicle Information: 2012 Volkswagen Passat TDI SE 2.0L

Issue + Repair History:

• Long history of check engine light being on.
• ECM not outputting a 5V reference on circuits for various sensors.
• Recent ECM replacement under warranty.
• Good powers and grounds to ECM confirmed.

Other OBD2 Codes + Test Results:

• No additional codes or test results mentioned.

Suggestions to Fix Issue:

• Check for any shorts in the sensor circuits that might be causing the ECM to shut down the 5V reference.
• Inspect all related wiring for potential shorts or damage.
• Consider potential issues outside of the ECM that could affect the 5V reference.

Final Fix:

• Found a short in the 5V reference circuit for the DEF pressure sensor.
• Short caused by trunk lid strut mount rubbing against the wiring.
• Repairing the wiring issue resolved the problem with the ECM’s 5V reference output.

Advice	Description of Diagnostic Step
Check for Shorted Sensor	Inspect sensors using the 5V reference line for potential shorts.
Inspect Wiring Harness	Examine the harness for damage or shorts affecting the 5V reference.
Verify ECM Output	Ensure the Engine Control Module is properly outputting the 5V reference.
Disconnect Sensors Individually	Isolate the issue by disconnecting sensors one at a time.
Load Test Circuits	Conduct load tests on circuits for hidden issues.
Check ECM Grounds and Power	Secure and check all ECM ground and power connections.
Consider ECM Replacement	If other diagnostics fail, the ECM might need replacement.