What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II. Although generic, specific repair steps may vary by make/model.
When I find a stored P0641 code, it means the Powertrain Control Module (PCM) has detected an open circuit for a particular sensor; designated in this case as “A”. When diagnosing an OBD-II code, the term open can be replaced with absent.
The sensor in question is typically associated with the automatic transmission, transfer case, or one of the differentials. This code will almost always be accompanied by a more specific sensor code. The P0641 adds that the circuit is open. Consult a reliable vehicle information source (All Data DIY is a great choice) to determine the location (and function) of the sensor regarding the vehicle in question. If P0641 is stored alone, suspect that a PCM programming error has occurred. Obviously, you will need to diagnose and repair any other sensor codes before diagnosing and repairing the P0641 – but keep the “A” open circuit in mind.
A Reference Voltage
A reference voltage (usually five volts) is applied to the sensor in question via a switched circuit (powered with the key on). There should also be a ground signal. The sensor is likely a variable resistance or electromagnetic type and completes a particular circuit. The sensor’s resistance decreases as pressure, temperature, or speed increases and vice versa. As the sensor’s resistance changes with conditions, it provides the PCM with an input voltage signal. If this input voltage signal is not received by the PCM, the circuit is considered open and a P0641 will be stored.
A Malfunction Indicator Lamp (MIL) may also be illuminated, but keep in mind that some vehicles require multiple drive cycles (with a failure) for an MIL to illuminate. For this reason, you should allow the PCM to enter readiness mode before considering a repair successful. Simply clear the code, once repairs are made, and drive the vehicle normally. If the PCM enters readiness mode, the repair was successful. If the code resets, the PCM will not enter readiness mode and you will know you still have a problem.
Severity and Symptoms
The severity of a stored P0641 depends on which sensor circuit is experiencing an open condition. Other stored codes must be considered before the severity can be determined.
Symptoms of a P0641 code may include:
- Transmission failure to switch between sport and economy modes
- Transmission shifting malfunctions
- Delayed (or no) transmission engagement
- Transmission failure to switch between four-wheel drive and two-wheel drive modes
- Transfer case failure to shift from low to high range
- Lack of front differential engagement
- Lack of front hub engagement
- Erratic or inoperative speedometer/odometer
Possible causes of this engine code include:
- Open circuits and/or connectors
- Faulty or blown fuses and/or fuse links
- Faulty system power relay
- Bad sensor
An Example of a P0641 Fault in the Wiring Harness
Diagnostic and Repair Procedures
To diagnose a stored P0641 code, I would need access to a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source (like All Data DIY). A portable oscilloscope can also be helpful in some circumstances.
Use your vehicle information source to determine the location and function of the sensor in question, regarding your specific vehicle. Check system fuses and fuse links with the circuit under full load. Fuses that may seem normal when there is very little load on the circuit often fail when the circuit is fully loaded. Blown fuses should be replaced, keeping in mind that a short circuit is likely the cause of the blown fuse.
Perform a visual inspection of wiring harnesses and connectors related to the sensor system. Repair or replace damaged or burnt wiring, connectors, and components as needed.
Next, I would connect the scanner to the vehicle’s diagnostic connector and retrieve all stored trouble codes. I like to write them down, along with any associated freeze frame data, as this information can prove useful if the code turns out to be intermittent. After that, I would go ahead and clear the code and test drive the vehicle to see if it immediately reset.
If all system fuses are intact and the code immediately resets, use the DVOM to test the reference voltage and ground signals at the sensor in question. Typically, you should expect to find five volts and a common ground at the sensor connector.
If voltage and ground signals are present at the sensor connector, proceed by testing the sensor’s resistance and continuity levels. Use your vehicle information source to obtain test specifications and compare your actual results with them. Sensors that do not meet these specifications should be replaced.
Disconnect all associated controllers from the system circuits before testing resistance with the DVOM. If there is no reference voltage signal at the sensor, disconnect all associated controllers and use the DVOM to check circuit resistance and continuity between the sensor and the PCM. Replace open or shorted circuits as necessary. If an electromagnetic sensor is used, with an alternating signal, use the oscilloscope to monitor live data; paying close attention to glitches and completely open circuits.
Additional Diagnostic Notes:
- This type of code is usually provided as support for a more specific code
- A stored P0641 code is normally associated with the transmission