Error Code P0642: What It Means & What To Do?

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P0642 is a diagnostic trouble code (DTC) for "Sensor Reference Voltage “A” Circuit Low". This code means that the powertrain control module (PCM) has detected a low reference voltage signal for a particular sensor that has been given the designation "A". The engine control module (PCM) sends out a 5-volt reference signal to all applicable engine sensors. If the input voltage signal, received by the PCM, is lower than a programmed limit, a P0642 will be stored.

OBD-II Code P0642 is technically defined as a Sensor Reference Voltage "A" Circuit Low. This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to OBD-II equipped vehicles. It indicates that a fault has been detected in the sensor reference voltage “A” circuit. The control module has 2 internal 5-volt reference buses called 5-volt reference 1 and 5-volt reference 2. Each reference bus provided 5-volt reference circuit for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to the reference bus. The control module monitors the voltage on the 5-volt reference buses. If the input voltage signal, received by the PCM, is lower than a programmed limit, a P0642 will be stored. A malfunction indicator lamp (MIL) may be also illuminated.

What causes the problem with the Sensor Reference Voltage "A" Circuit Low?

• Open or shorted wiring or connectors
• Engine sensor failure
• Faulty or blown fuses
• Loose or disconnected ground wires to the PCM
• PCM failure

1. Sensor - Sensors are sophisticated devices that are frequently used to detect and respond to electrical or optical signals. A Sensor converts the physical parameter (for example:  temperature, blood pressure, humidity, speed, etc.) into a signal which can be measured electrically. To avoid any trouble code related to sensor, it should be checked thoroughly and repaired or replaced as required.
2. Transmission Parts - A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. Each and every component of the transmission should be checked carefully and replaced if required.
3. PCM - A powertrain control module, abbreviated PCM, is an automotive component, a control unit, used on motor vehicles. It is generally a combined control unit, consisting of the engine control unit (ECU) and the transmission control unit (TCU). PCM should also be checked as any faulty component of PCM can lead to trouble codes.
4. Connectors - Connector is an electro-mechanical device used to join electrical terminations and create an electrical circuit. Electrical connectors consist of plugs and jacks. Check out all the connectors carefully as these should not be loose, broken or corroded, else can create problems.

Below are the common symptoms for the code P0642. If you experience below mentioned symptoms please bring your vehicle to a reputable repair facility. Keep in mind; problems don’t get better over time unless rectified.

• Transmission may face problem while shifting
• Transmission may not switch between 2WD and AWD modes
• Transfer case may not shift between high and low ranges
• Lack of front differential engagement
• Defective Speedometer
• Decrease in engine power
• Decreased fuel efficiency
• Engine misfires

It is important to have the vehicle inspected and repaired right away once the P0642 code is discovered as it will affect the drivability of the vehicle and decrease the overall engine performance. Repairs and replacements that may help correcting this code are as follows:

• Look for the open or shorted ECM harness, repair or replace as required.
• Inspect the sensor carefully and replace the shorted sensor on 5-volt circuit.
• Replacing any control modules that are malfunctioning.
• Replace the faulty ECM.

It is also important that the P0642 code be cleared and the system be retested after every possible repair. This process narrows down the root cause of the issue until the correct repair is found and performed.

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Trouble code P0642 should be diagnosed and rectified at earliest to avoid any major issues in near future. Below mentioned are some common steps to diagnose this code:

• Perform a visual inspection of sensor system related wiring harnesses and connectors.
• Repair or replace damaged or burned wiring, connectors, and components as required.
• Second, connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and freeze frame data.
• Now you may go ahead and clear the code; then test drive the vehicle to see if it is immediately reset.
• Use the DVOM to test reference voltage and ground signals at the sensor in question.
• Continue by testing sensor resistance and continuity levels, if the voltage and ground signals are present at the sensor connector.
• If reference voltage is low (at the sensor), use the DVOM to check circuit resistance and continuity between the sensor and the PCM.
• Replace open or shorted circuits as necessary.

Common Mistakes When Diagnosing the P0642 Code

A miscommunication between control modules will affect the drivability of the vehicle, and decrease the overall engine performance and due to this miscommunication between control modules, other error codes arises in PCM. These are typically a result of the faulty connection causing the P0642 code, and do not actually require repair. However, it is common for these issues to be diagnosed and repaired first, which does not solve the underlying issue.