Powertrain, generic · Computer and auxiliary outputs

P0644 Driver Display Serial Communication Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P0644 concerns a dedicated serial line on which the engine module sends values or warnings to a driver display or information display – not the general vehicle CAN bus. The module reports missing or invalid communication on that line (open, short, display not answering); CARB requires digital outputs to be checked for communication faults. GM Europe's 2010 Saab, for example, passes engine data such as fuel level over CAN to the body module and on to the instrument unit and information display – where such data runs on the bus, a failure is reported as a U code (e.g. U0155) instead. No manufacturer document available to us uses this code; the description follows CARB and the component. Displays then show missing, frozen or jumping information; the engine runs normally.

Likely causes

Wire or connector between module and receiver open, corroded or shorted
Display or instrument cluster faulty or without supply (fuse, ground)
Low system voltage during cranking
Output driver in the module faulty

Symptoms

  • Warning message or lamp (depending on the maker)
  • Information in the driver display missing, frozen or jumping
  • Display stays dark

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

driver displayserial data lineengine control module

Related codes

P0600U0155P0650P0656P0654

References: law.cornell.edu, saabwisonline.com.