P0644 code definition
The P0644 trouble code detects a communication error between multiple control modules
What the P0644 code means
The P0644 code is a standard OBD-II trouble code that notes a control module communication problem. The powertrain control module (PCM) is in constant communication with the other control modules, which include the climate control module, anti-lock brake control module, transmission control module, and traction control module, among others. If the PCM notes a failure of communication between itself and another control module, or if the PCM is unable to control a supporting control module, then the P0644 code will be noted.
What causes the P0644 code?
Causes for the P0644 trouble code include:
- Faulty electrical connection in the PCM
- Defective PCM driver
- Loose control module ground strap
- Broken ground wire
- Damaged electrical components in the controller area network (CAN) bus
- In rare cases, a defective PCM or CAN bus
What are the symptoms of the P0644 code?
The P0644 trouble code will usually be accompanied by an illuminated Check Engine Light. It is common for the vehicle to have engine and transmission performance issues, such as delayed acceleration, harsh shifting, rough idling, and frequent stalling at idle. Reduced fuel efficiency is also common.
How does a mechanic diagnose the P0644 code?
The P0644 code will be diagnosed using an OBD-II trouble code scanner. A certified mechanic will use the scanner to view the freeze frame data and gather information about the P0644 code. Additional trouble codes will also be searched for, and, if found, should be addressed in the order in which they are displayed on the scanner. The mechanic will then need to reset the trouble codes, restart the vehicle, and see if the P0644 code remains detected. If the code has disappeared, then an intermittent error or an inaccurate detection are likely to blame.
If the code remains detected, then the mechanic will begin by visually inspecting the electrical components in the PCM and CAN bus. Any loose or corroded connectors, shorted wires, or blown fuses will require replacement. The ground strap and ground wire will then need to be tested.
Next, the mechanic can hook up a specialized diagnostic CAN scanner, which provides detailed information about the control modules and circuitry. This scanner can be used to pinpoint where the problem that is causing the P0644 code is coming from.
Whenever the mechanic replaces any part, the codes should again be reset, the vehicle again restarted, and the P0644 code searched for. This helps the mechanic identify as soon as the error has been resolved.
Common mistakes when diagnosing the P0644 code
The most frequently made mistakes when diagnosing the P0644 trouble code come from a failure to properly follow the OBD-II diagnosis protocol. It is imperative that technicians always comply with the protocol, as this helps ensure a speedy and accurate diagnosis.
Numerous other trouble codes are usually present when the P0644 code is detected. Often, these codes are simply a reaction to a communication failure created by the P0644 code. If the codes are addressed out of order, a reactionary code may be erroneously inspected, and unnecessary repairs may take place.
How serious is the P0644 code?
A vehicle with the P0644 trouble code will likely still be drivable, though there will usually be noticeable drivability issues. An inspection should be scheduled as soon as the code is detected.
What repairs can fix the P0644 code?
Repairs for the P0644 trouble code are:
- Replacement of electrical connections in the PCM
- Replacement of electrical components in the CAN bus
- Update of the PCM drivers
- Replacement of the ground strap or ground wire
- In rare cases, replacement of the PCM or CAN bus
Additional comments for consideration regarding the P0644 code
It is uncommon for the P0644 trouble code to occur because of a defective PCM or CAN bus. Mechanics should always inspect all other potential causes before presuming that there is a PCM or CAN bus failure. If either of these two components does need to be replaced, it will also need to be reprogrammed.
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