Technical Description of OBD-II Trouble Code
Low Circuit for Sensor Reference Voltage “A”
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
If your OBD-II equipped vehicle has a stored P0642 code, it means that the powertrain control module (PCM) has detected a low reference voltage signal for a specific sensor designated as “A”. This sensor is typically associated with the automatic transmission, transfer case, or one of the differentials.
In most cases, there will be another specific sensor code accompanying the P0642 code. The P0642 code indicates that the sensor reference circuit voltage is low. To determine the location and function of the sensor in question for your vehicle, consult a reliable source of vehicle information such as All Data DIY. If the P0642 code is stored alone, it may indicate a PCM programming error. However, it is important to diagnose and repair any other sensor codes before addressing the P0642 code, while keeping the low reference voltage condition in mind.
The sensor in question receives reference voltage (typically five volts) through a switched circuit that is energized with the key on. There will also be a ground signal. The sensor, which can be of the variable resistance or electromagnetic type, completes the circuit. As pressure, temperature, or speed changes, the sensor resistance should decrease or increase accordingly. These resistance changes provide the PCM with an input voltage signal.
If the input voltage signal received by the PCM is lower than the programmed limit, a P0642 code will be stored. In some cases, a malfunction indicator lamp (MIL) may also illuminate. Some vehicles may require multiple drive cycles with a failure for the MIL to illuminate. It is important to allow the PCM to enter readiness mode before considering any repair successful. After performing the necessary repairs, simply clear the code and drive the vehicle normally. If the PCM enters readiness mode, it indicates that the repair was successful. If the code is reset, the PCM will not enter readiness mode, indicating that a malfunction still exists.
Severity & Symptoms
The severity of a stored P0642 code depends on which sensor circuit is experiencing the low voltage condition. It is important to consider any other stored codes before determining the severity.
Symptoms of a P0642 code may include:
Causes
Possible causes for this engine code include:
Diagnostic and Repair Procedures
To diagnose a stored P0642 code, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information such as All Data DIY. A portable oscilloscope may also be helpful in the diagnosis.
First, consult your vehicle information source to determine the location and function of the sensor in question for your specific vehicle. Perform a visual inspection of the wiring harnesses and connectors related to the sensor system. Repair or replace any damaged or burned wiring, connectors, and components as necessary. Next, connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and freeze frame data. Write down the codes, the order in which they were stored, and any related freeze frame data, as this information may be useful if the code is intermittent. Clear the code and test drive the vehicle to see if it is immediately reset.
If the code is immediately reset, use the DVOM to test the reference voltage and ground signals at the sensor in question. Normally, you would expect to find five volts and a ground at the sensor connector.
Proceed to test the sensor resistance and continuity levels if the voltage and ground signals are present at the sensor connector. Obtain the testing specifications from your vehicle information source and compare your actual findings to them. Sensors that do not comply with these specifications should be replaced.
Before testing the resistance with the DVOM, disconnect all related controllers from the system circuits. Failure to do so could result in PCM damage. If the reference voltage is low at the sensor, use the DVOM to check the circuit resistance and continuity between the sensor and the PCM. Replace any open or shorted circuits as necessary. If the sensor in question is electromagnetic with a reciprocating signal, use the oscilloscope to monitor live data and focus on glitches and completely open circuits.
Additional diagnostic notes:
