P0642 Low voltage sensor reference circuit

OBD-II Trouble Code - P0642 - Data Sheet

P0642 - Low voltage in the reference voltage circuit of the sensor "A".

Code P0642 indicates that the "A" sensor reference voltage circuit is too low compared to manufacturer's specifications as detected by the transmission control module or PCM.

What does trouble code P0642 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles. Although general, specific repair steps may differ depending on the brand / model.

If your OBD II vehicle has a P0642 stored, it means that the powertrain control module (PCM) has detected a low reference voltage signal for a specific sensor, which is designated "A". The sensor in question is usually associated with an automatic transmission, transfer case, or one of the differentials.

A more specific sensor code almost always accompanies this code. P0642 adds that the sensor reference circuit voltage is low. To determine the location (and function) of the sensor for a particular vehicle, consult a trusted vehicle information source (All Data DIY is a great option). I suspect a PCM programming error has occurred if P0642 is stored separately. You will need to diagnose and repair any other sensor codes prior to diagnosing and repairing P0642, but be aware of the low reference voltage.

The sensor in question is supplied with a reference voltage (usually five volts) through a switchable (powered when the switch is on) circuit. There will also be a ground signal. The sensor will be either variable resistance or electromagnetic type and it completes the circuit. The resistance of the sensor should decrease with increasing pressure, temperature or speed, and vice versa. As the sensor resistance changes (depending on conditions), it supplies the PCM with an input voltage signal.

If the input voltage signal received by the PCM is below the programmed limit, P0642 will be stored. A malfunction indicator lamp (MIL) may also be illuminated. Some vehicles will require several driving cycles (in case of failure) for the warning lamp to illuminate. Let the PCM go into readiness mode before assuming a repair is successful. Just remove the code after repair and drive as normal. If the PCM goes into readiness mode, the repair was successful. If the code is cleared, the PCM will not go into standby mode and you know the fault is still there.

Severity and symptoms

The severity of the stored P0642 depends on which sensor circuit is in the low voltage state. Other stored codes must be reviewed before a severity determination can be made.

In addition to having a P0642 code in the PCM memory, the vehicle may not start. Starting may be difficult and the engine may run rough. Fuel efficiency may decrease, the engine may misfire, and the driver may notice a general reduction in engine power. The Check Engine light will come on, but this may take several driving cycles.

Symptoms of a P0642 code may include:

• Inability to switch the transmission between sport and economy modes
• Gear shift malfunctions
• Delay (or lack) of turning on the transmission
• Transmission failure to switch between XNUMXWD and XNUMXWD
• Failure of the transfer case to switch from low to high gear
• Lack of inclusion of the front differential
• Lack of engagement of the front hub
• Incorrect or not working speedometer / odometer

Causes of the P0642 code

In most cases, code P0642 is caused by a short or open in the wiring or connectors between the supporting control modules or from the control modules to the PCM. Other reasons may include:

• Defective engine control module (ECM)
• Open or short circuit of electrical components engine sensors
• Open or short in ECM harness
• Short circuit of the sensor on a 5-volt circuit
• Loose or disconnected ground wires in PCM input circuit
• Internal faults in PCM
• Bad sensor
• Defective or blown fuses and / or fuses
• Faulty system power relay
• Open circuit and / or connectors

Diagnostic and repair procedures

Diagnosing a stored P0642 code will require a diagnostic scanner, a digital volt / ohmmeter (DVOM), and a trusted source of vehicle information (such as All Data DIY). A handheld oscilloscope can also be helpful in making a diagnosis.

First, consult your vehicle information source to determine the location and function of the sensor in question, as it relates to your specific vehicle. Visually inspect the harness and connectors associated with the sensor system. Repair or replace damaged or burnt wiring, connectors, and components as needed. Secondly, connect the scanner to the vehicle diagnostic port and retrieve all stored DTCs and freeze frame data. Make a note of the codes along with the order in which they were stored and any relevant freeze frame data, as this information may be helpful if the code turns out to be intermittent. Now you can go ahead and clean up the code; then test drive the vehicle to make sure it is reset immediately.

If the code resets immediately, use the DVOM to test the reference voltage and ground signals on the sensor in question. Typically you would expect to find five volts and ground at the sensor connector.

Continue testing sensor resistance and continuity levels if voltage and ground signals are present at the sensor connector. Obtain test specifications from your vehicle information source and compare your actual results to them. Sensors that do not meet these specifications should be replaced.

Disconnect all related controllers from the system circuits before testing resistance with the DVOM. Failure to do so could damage the PCM. If the reference voltage is low (at the sensor), use the DVOM to test the circuit resistance and continuity between the sensor and the PCM. Replace open or shorted circuits as necessary. If the sensor in question is a reciprocating electromagnetic sensor, use an oscilloscope to track the data in real time. Focus on crashes and completely open circuits.

Additional diagnostic notes:

• This type of code is usually provided as support for more specific code.
• A stored code P0642 is usually associated with the transmission.

Common Mistakes When Diagnosing Code P0642

In many cases, when the problem is with the communication between the control modules, other codes appear in the PCM. This is usually the result of a bad connection causing a code P0642 and does not actually require repair. However, these problems are usually diagnosed and fixed first, which does not solve the underlying problem.

How serious is the P0642 code?

If an OBD-II scanner detects code P0642, it is important to have the vehicle checked and repaired immediately. A problem between the control modules will affect the vehicle's handling and reduce the overall performance of the engine. To make sure the car stays in working order, it's important to fix the problem that's causing the P0642 code right away.

What repairs can fix code P0642?

In many cases, the cause of a code P0642 will be corrected during the initial diagnosis, as the most likely problem with this code being stored is electrical components. However, there are other types of repairs that may be required to resolve the underlying problem. These include:

• Replacing a failed engine control module (ECM).
• Replacing or repairing an open or shorted ECM harness.
• Replacing a shorted sensor with a 5-volt circuit.
• Replacing the PCM with signs of internal problems.
• Replacing faulty control modules.

If any control modules are replaced, 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 driving cycles. An incorrectly programmed PCM can result in the vehicle not starting.

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

P0642 ✅ SYMPTOMS AND CORRECT SOLUTION ✅ - OBD2 Fault Code

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