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P codepowertrainLow severity

P0644Driver Display Serial Communication Circuit

The serial data communication circuit between the PCM and the driver's display or instrument cluster has a fault.

Definition

The PCM shares engine data with the instrument cluster or driver information display over a dedicated serial data line or network circuit, and it monitors that link for valid activity. P0644 sets when the communication circuit is open, shorted, or the expected data handshake fails. Symptoms include inoperative or erratic gauges (tachometer, temperature, trip computer data), warning messages, or a blank driver display, while the engine itself typically runs normally. The MIL may illuminate depending on the application, and the loss of displayed warnings is the main practical concern.

Common causes

  • Open, chafed, or shorted wiring in the serial data circuit between PCM and cluster
  • Corroded or backed-out terminals at the instrument cluster or PCM connector
  • Failing instrument cluster/driver display module loading down the data line
  • Poor cluster power or ground causing the module to drop off the data link
  • Aftermarket accessories (remote start, gauges) tapped into the data circuit
  • Faulty PCM serial communication driver (least common)

Common misdiagnoses

  • Replacing the instrument cluster when a connector terminal or splice was at fault
  • Replacing the PCM before testing the data circuit wiring
  • Replacing gauge sending units for what is a data-link problem
  • Blaming the battery or ignition switch for a display dropout on the serial circuit

Troubleshooting steps

  1. 1.Verify the symptom and scan all modules

    Note exactly which display functions are dead and check for companion U-codes (such as lost communication with the cluster) that confirm a data-link fault rather than a gauge fault.

  2. 2.Check cluster power and ground

    Verify battery voltage on the cluster's feed circuits with the key on and less than 0.1 volt drop on its grounds, since a browned-out module stops communicating.

  3. 3.Test the serial data circuit

    With the appropriate connectors accessed, check the data wire for continuity end to end, shorts to ground and voltage, and (on bus circuits) the specified resistance or bias voltage per the service manual.

  4. 4.Disconnect aftermarket devices

    Unplug any remote start, alarm, or accessory tapped near the cluster harness and retest, since added devices commonly load or interrupt the data circuit.

  5. 5.Substitute or bench-test the display module

    If wiring and power check good, a known-good cluster or a scan-tool activation test isolates whether the cluster or the PCM side of the link has failed.

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