Fix P0642: Mechanic’s solutions that WORKED

The OBD2 code P0642 refers to “Sensor Reference Voltage ‘A’ Circuit Low,” indicating that the Engine Control Module (ECM) has detected a low voltage condition on the 5-volt reference circuit. This code is critical because the 5-volt reference circuit is used by multiple sensors to communicate with the ECM. A low voltage condition can affect various sensor readings, leading to poor engine performance, stalling, or other drivability issues.

P0642 quick overview

Meaning	P0642: Sensor Reference Voltage “A” Circuit Low
Is it serious?	Yes, this code indicates a fundamental electrical issue that can affect the vehicle’s performance and drivability.
Possible causes	– Faulty sensor or sensor circuit – Water intrusion in electrical connectors – Faulty ECM – Wiring short or open in the 5-volt reference circuit
How to diagnose?	Check for water intrusion in connectors of sensors using the 5V reference. Inspect wiring harness for damage, corrosion, or shorts. Unplug sensors one at a time to identify a potential faulty sensor. Check the ECM and PCM for proper operation and reference voltage output. Investigate related components like the fuel pressure sensor, EGR valve, and accelerator pedal position sensor based on associated trouble codes.

How to fix P0642 – learn from mechanics

I looked into P0642 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: Ford

Vehicle Information 2006 Ford Explorer Eddie Bauer 4.6L

Issue + Repair History

• Vehicle was running rich and stalling.
• Replaced Fuel Rail Pressure Sensor.
• Reset control unit.
• Continuously experiencing P0642 trouble code.

Other OBD2 Codes + Test Results

• P0191 – Fuel Rail Pressure Sensor
• P0642 – Sensor Reference Voltage “A” Low
• Found water in connector of high-pressure A/C switch causing loss of reference voltage.
• Cleaned harness side and replaced switch.

Mechanic Suggestions

• Unplug sensors using the 5-volt reference until the reference voltage comes back.
• If everything is unplugged and the issue persists, inspect the wiring harness.
• Consider disconnecting the 5-volt reference at the PCM to diagnose further.
• Check the FPDM above the rear fuel tank for any damage or water ingress.

Final Fix

• Cleaned harness side and replaced the high-pressure A/C switch, resolving the issue.

Case #2: Chevrolet

Vehicle Information 2005 Chevrolet Silverado 2500 HD 6.6L

Issue + Repair History

• Truck experiencing stalling issues intermittently.
• Only trouble code returning is P0642.
• Replaced accelerator pedal position sensor.

Other OBD2 Codes + Test Results

• P0642 – 5 Volt Ref 1 Circuit Low Voltage
• P0700 – TCM Request MIL On
• P1404 – EGR Closed Position Performance
• Identified a recall related to computer numbers 5E275688 THRU 5E276640 and 5F925227 THRU 5F928038 involving various components.
• Traced a 5V supply from PCM to the Turbo Vane Position Sensor.
• Suspected EGR valve stuck open.

Mechanic Suggestions

• Refer to TSB reference #05539A dated 4/27/05 regarding the recall.
• Unplug each item one at a time until the P0642 code disappears to identify the faulty component.
• Investigate further if all items are unplugged and the code persists, indicating potential wiring shorts or a faulty PCM.
• Address the transmission control module code to diagnose transmission issues.

Final Fix

• Replaced the accelerator pedal position sensor, resolving the stalling issue.

Case #3: Dodge

Vehicle Information 2007 Dodge Charger R/T 5.7L

Issue + Repair History

• MIL on with harsh transmission engagement.
• Codes found in various modules: cluster, steering, ABS, and transmission.
• P0642 code recurring in transmission module every two key cycles.
• 5-volt reference to throttle body, shared with oil sender, crank sensor, and APP, showed fluctuating spike but remained constant overall.
• Previous repairs include replacement of ECM, oil sender unit, and throttle body with a used one.

Other OBD2 Codes + Test Results

• P0642 – Sensor Reference Voltage 1 Circuit Low
• Tested the 5-volt reference circuit, which showed a consistent 5-volt reference.

Mechanic Suggestions

• Investigate spikes in the 5-volt reference circuit, considering potential interference from sources like overhead lights or ignition circuit leaks.
• Check for voltage drop across power and ground connections, ensuring they are within acceptable limits.
• Verify the VREF for 6V in the TCM and its supply to various sensors, ensuring it falls within the specified range of 4.8V to 7.2V.

Final Fix

• Identified a bad transmission as the root cause of the P0642 code, although the repair was declined by the customer.

Diagnostic Step	Explanation
Check for water in connectors	Look for moisture that could cause electrical shorts in sensor connectors.
Unplug sensors using the 5V reference	Identify if a specific sensor is causing the voltage drop.
Inspect the wiring harness	Search for any damage or signs of wear that could cause electrical issues.
Refer to technical service bulletins (TSBs) and recalls	Some vehicles may have known issues with specific components that are addressed by the manufacturer.
Check related OBD2 codes	Additional codes can provide clues to the underlying problem.
Replace suspected faulty components	Based on diagnostics, replace components like the accelerator pedal position sensor or high-pressure A/C switch.

These cases highlight the importance of thorough diagnostics when dealing with electrical issues in vehicles. Each case presents a unique challenge but also underscores common steps such as checking for water intrusion, inspecting the wiring harness, and systematically testing components until the problem is identified and resolved. Remember, while some fixes might seem straightforward, electrical problems can be complex and often require professional expertise to ensure they are properly diagnosed and repaired.