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Error Code P0642 is defined as Sensor Reference Voltage “A” Circuit Low, meaning there’s an abnormally low voltage reading detected within the “A” circuit, likely caused nu faulty engine control module or other malfunctions.
This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and power configuration.
When Error Code P0642 is stored, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected a reference voltage that is abnormally low for a particular sensor that has the designation “A.” The sensor in question is usually associated with the automatic transmission, one of the differentials, or transfer case.
This code also almost always comes with a more specific sensor code. Error Code P0642 simply adds that the reference circuit voltage is low. Thus, make sure you consult with your reliable vehicle source information to determine the location (and function) of that sensor. If this code is stored, this means the PCM programming error has occurred. This, of course, requires you to diagnose and repair other sensor codes prior to diagnosing and repairing Error Code P0642 – keep the open “A” circuit in mind.
The reference voltage (usually 5V) is applied to the sensor in question through a switched (energized with key on) circuit. Also, there must be a ground signal. The sensor is maybe of either the variable resistance or electromagnetic variety and completes a specific circuit. Sensor resistance goes down as pressure, temperature, or vehicle speed increases, and vice versa. As the sensor resistance changes with the conditions, it provides the PCM with an input voltage signal. If the PCM does not receive this input voltage signal, then the circuit is considered open, and this code will be set.
If the PCM receives an input voltage signal that is lower than expected, then Error Code P0642 will be set, and simultaneously activate the Check Engine light.
For some vehicles, it may require multiple drive cycles before the Check Engine light is activated. For this, you must allow the PCM to enter readiness mode before addressing the problem.
Thus, simply clear the code after repairs are made, and then drive the vehicle normally. If the PCM enters readiness mode, then that means the repair was successful.
If the code resets, on the other hand, this means the PCM is unable to enter readiness mode, and you still have to proceed on your diagnosis.
Symptoms of this code vary on its severity, particularly on the extent of the open condition. Some of this code’s common symptoms include:
- Malfunction of transmission shifting
- Failure of transmission to shift from sport to economy mode
- Delayed or no transmission engagement
- Failure of transfer case to shift from low to high gear
- Failure of the transmission to switch between all-wheel and two-wheel drive modes
- Lack of front hub engagement
- Lack of front differential engagement
- Inoperative or erratic speedometer/odometer
There are many causes of this code, such as:
- Open in the circuits and connectors
- Failed system power relay
- Blown or defective fuses and fusible links
- Defective sensor
How to Check
As with the majority of other error codes, diagnosing this code require a diagnostic scanner, DVOM (digital volt/ohmmeter), and a dependable vehicle source information. A portable oscilloscope can be of help under certain circumstances.
Refer to the vehicle information source to locate and understand the function of the sensor in question. Check all the connectors and wirings harnesses related to the sensor system. Repair or replace damaged wires as necessary.
Next, connect your diagnostic scanner to the vehicle’s diagnostic connector, get all stored trouble codes too. You can write these codes down, including their freeze frame data, as this information can be useful for you. Then, clear the code and then take the vehicle for a test drive to see if the vehicle comes back.
If the code does NOT come back (doesn’t reset), then you have an intermittent condition, which means you have to wait for the problem to get worse first before you can successfully diagnose and conduct the repair.
If the code comes back (reset) on the other hand, then you can continue with your diagnosis.
Test for reference voltage and ground signals at the sensor in question using the DVOM. There should be about 5V and a ground at the sensor connector. If the voltage and ground signals are both present, then test for resistance and continuity levels. Get the specifications for testing from your vehicle information source and compare your actual findings to them. Sensors that are unable to comply with the specifications set by the manufacturer must be replaced.
Next, unplug al related controllers from the system circuits before testing resistance with the DVOM. If the reference voltage is lower (at the sensor), then check circuit resistance and continuity between the sensor and PCM using the DVOM. Replace open or shorted circuits as needed.
If the vehicle uses an electromagnetic sensor that comes with a reciprocating signal, then you can use the oscilloscope to monitor live data. Make sure you pay close attention to the glitches and circuits that are completely open.
How to Fix
Depending on your diagnosis, common repairs for this code include:
- Repair or replace damaged, burnt, chafed wiring harness and connectors
- Replacement of blown fuses
- Replacement of sensor
- Replace open or shorted circuits
Error Code P0642 is usually associated with the drivetrain and comes as support for a more specific error code.