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P0642 OBD-II Trouble Code: Sensor Reference Voltage “A” Circuit Low

check engine light

P0642 code definition

The P0642 code indicates that the sensor reference voltage “A” circuit is too low compared to the manufacturer’s specifications, as detected by the powertrain control module, or PCM.

What the P0642 code means

The PCM or one of the supporting control modules has detected that there is a voltage reading that is too low in a particular area of the circuit related to one of many vehicle components, including climate control, proximity alert, traction control, turbo control, cruise control, anti-theft features, fuel injection control, antilock brake control, and others.

What causes the P0642 code?

Most of the time, the P0642 code is caused by shorted or open wiring or connectors between the supporting control modules, or from the control modules to the PCM. Other causes may include:

  • Faulty engine control module (ECM)
  • Open or shorted electrical components from the engine sensors
  • Open or shorted ECM harness
  • Shorted sensor on 5-volt circuit
  • Loose or disconnected ground wires in the PCM input circuitry
  • Internal malfunctions in the PCM

What are the symptoms of the P0642 code?

In addition to the presence of the P0642 code in the PCM memory, the vehicle may not be able to start. Starting may be difficult, and the engine may run roughly. Fuel efficiently may decrease, the engine may misfire, and the driver may notice an overall decrease in engine power. The Check Engine Light will be illuminated, but it may take several drive cycles to do so.

How does a mechanic diagnose the P0642 code?

After an OBD-II scanner verifies the P0642 code, a technician should visually inspect all the wiring, connectors, and other components related to the PCM and the supporting control modules. Any damaged elements should be replaced, reconnected, or repaired as necessary. The technician should clear the code from the PCM after ensuring that all electrical components are working properly, and perform a test drive. If the code returns, the issue requires more in-depth repair.

Common mistakes when diagnosing the P0642 code

In many cases when the issue is related to communication between control modules, other codes will appear in the 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.

How serious is the P0642 code?

If an OBD-II scanner detects the P0642 code, it is important to have the vehicle inspected and repaired right away. A miscommunication between control modules will affect the drivability of the vehicle, and decrease the overall engine performance. In order to ensure that the vehicle remains operational, it’s important to address the issue causing the P0642 code right away.

What repairs can fix the P0642 code?

In many cases, the cause of the P0642 code will be repaired during the initial diagnosis, because the electrical components are the most likely issue when this code is stored. However, there are other repairs that may need to be made to solve the root issue present. These include:

  • Replacing faulty engine control module (ECM).
  • Replacing or repairing open or shorted ECM harness.
  • Replacing shorted sensor on 5-volt circuit.
  • Replacing a PCM that shows signs of internal malfunctions.
  • Replacing any control modules that are malfunctioning.

When replacing any control modules, the new modules will have to be reprogrammed. If the mechanic skips this step, it is likely that many codes will be stored over the next few drive cycles. A PCM that is not properly programmed can make the vehicle enter a no start condition.

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.

Need help with a P0642 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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