Error Code P0644 is defined as Driver Display Serial Communication Circuit, which means there’s an error detected in the communication between multiple control modules, likely caused by a faulty driver or bad electrical connection in the PCM.

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. This includes models from Audi, BMW, Ford, General Motors, Jaguar, Mercedes Benz, Volkswagen, etc.  Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and power configuration.

When Error Code P0644 appears, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected a malfunction in the serial data control circuit of the driver display system.

For most vehicles, the driver display refers to the dash-mounted LCD monitor. For some vehicles, it is the heads-up display that appears on the driver’s side windshield.

Because of the limitations in cost and space associated in manufacturing most vehicles, the communication between the PCM and other onboard controllers is a serial data communication. This means bytes of data are sequentially transferred, one byte at a time.

All of the onboard serial data communication is carried out through the CAN (controller area network), which is basically an assembly of connectors and wirings inside the harness. It’s a uniform for a specific automobile type, making it possible to use a common set of controllers, actuators, sensors, and a single type of diagnostic system (OBD-II).

If the PCM (or any other onboard controllers) determines a malfunction in the driver display serial data communication circuit, then Error Code P0644 will be stored, while simultaneously activate the Check Engine light. For some vehicles, it may require multiple drive cycles for the Check Engine light to activate.

Common Symptoms

Vehicles with engine and transmission performance issues result in a wide variety of drivability problems, such as:

  • Delayed acceleration
  • Rough idle
  • Harsh shifting
  • Frequent stalling at idle
  • Increase fuel consumption
  • Loss of driver display system

Other stored codes may be present as well.

Possible Causes

  • Shorted or open connectors or wiring in the CAN bus
  • Blown fuse or fusible link
  • Defective driver display monitor
  • Defective relay
  • Bad or programming error in PCM

How to Check

Serial data communication codes can be challenging to diagnose accurately, but there are several tools that can definitely help for preliminary tests. These tools are pretty basic, namely diagnostic scanner and vehicle information source.

The vehicle information source is for the TSB (technical service bulleting) of the fault code, or at least factors parallel to the condition, such as the vehicle, engine size, codes stored, symptoms, etc. If you can find applicable TSB information for this code, then it helps for a more accurate diagnosis.

Start your diagnosis by connecting the diagnostic scanner to the vehicle’s diagnostic port. Retrieve all codes stored, including freeze frame data. Write this information down. Then, clear the codes and then take the vehicle for a test drive until one of the two scenarios happen:

  1. The code does NOT reset, and the PCM enters readiness mode
  2. The code resets

If the first scenario happens, then you have an intermittent code condition, which means you will have to wait for the problem to get worse before you can diagnose it more accurately.

If the second scenario happens, then you need to inspect all related connectors and wirings. Repair as needed

Check the PCM power supply fuses and relays as well, and of course, repair or replace any damaged component as needed.

If you find no problems in your visual inspection, then you will have to test controller grounds, and repair as needed.

How to Fix

Depending on your diagnosis, common repairs for this code include:

  • Repair or replacement of damaged or faulty connectors and wirings
  • Replacement of electrical connections in the PCM
  • Replacement of electrical components in CAN bus
  • Replacement of ground strap or ground wire
  • Update PCM drivers
  • Replacement of PCM or CAN bus (rare)

If the PCM has both ground and voltage at the right terminals, then you can suspect damaged or crashed monitor, or PCM programming error. In which case, use a DVOM (digital volt ohmmeter) to test voltage and ground to the PCM and other controllers.