What does it mean?
This is a generic diagnostic trouble code (DTC) and applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Audi, BMW, General Motors, Ford, Jaguar, Mercedes-Benz, Volkswagen, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If a P0644 code has been stored in your OBD-II equipped vehicle, it means that the Powertrain Control Module (PCM) has detected a malfunction in the serial data circuit of the driver display system.
In most cases, the driver display refers to the LCD screen mounted on the dashboard, but it can also refer to the heads-up display that appears on the windshield on the driver’s side of some vehicles.
Due to space and cost limitations associated with mass-produced vehicles, communication between the PCM and other onboard controllers is serial data communication. This means that unique bytes of data are transferred sequentially—one byte at a time.
All onboard serial data communications are performed using the Controller Area Network (CAN). The CAN is a conglomeration of wiring and connectors that is formed into a harness. It is uniform for a particular type of automobile. Therefore, it is possible to use a common set of controllers, sensors, and actuators, as well as a single type of diagnostic system (OBD-II).
If the PCM (or one of the other onboard controllers) detects a malfunction in the driver display serial data communication circuit, a P0644 code will be stored, and a Malfunction Indicator Lamp (MIL) may illuminate. In most cases, the MIL illumination will require more than one ignition cycle in case of a failure.
A dashboard with multiple LCD screens:
How severe is this DTC?
This type of serial data communication code should not be accompanied by driving or safety issues. However, a P0644 code could be associated with a loss of the driver display. This could be inconvenient but would not be classified as a severe condition.
What are some of the symptoms of the code?
Symptoms of a P0644 trouble code may include:
Loss of the driver display system
Other stored codes
What are some of the common causes of the code?
Causes
of this code may include:
Short-circuited or open wiring or connectors in the CAN bus
Faulty driver display monitor
Blown fuse or fuse link
Faulty relay
Incorrect PCM or programming error
What are the troubleshooting steps for P0644?
Although serial data communication codes can be very difficult to diagnose accurately, there are several tools that will be necessary to perform some preliminary tests.
A diagnostic scanner and a reliable source of vehicle information will be helpful in attempting to diagnose a P0644 code.
You can use the vehicle information source to look for Technical Service Bulletins (TSBs) that parallel the vehicle, engine size, symptoms presented, and stored codes in your situation. If you find an applicable TSB, it may help you diagnose the P0644 code more quickly. When dealing with a serial data communication code, finding the applicable TSB may be the best thing that can come out of the situation.
Start by connecting the scanner to the vehicle’s diagnostic port and retrieving all stored codes and relevant freeze frame data. Note this information in case you need it later (once the codes are cleared). Then, clear the codes and test the vehicle until one of two scenarios occurs:
A. The code does not reset and the PCM enters readiness mode
B. The code is reset
If scenario A occurs, you are dealing with an intermittent code, and the conditions that caused it may need to worsen before an accurate diagnosis can be made.
If scenario B occurs, perform a visual inspection of all associated wiring and connectors. Check the PCM power fuses and relays. Perform necessary repairs. If no issues are detected, test the controller grounds and repair them if necessary.
If the PCM has proper ground and voltage at the terminals, suspect that the monitor has failed or there is a programming/PCM error.
Use the DVOM to test the voltage and ground of the PCM and other controllers