Powertrain, generic (extended)

P23A9 Communication Fault - Forced Engine Shutdown

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

P23A9 records that the engine was stopped, or kept from running, because the engine module lost contact with a control unit whose messages it needs to operate the engine safely. The title names only the consequence; which data link is meant is defined by each manufacturer, and none publishes P23A9. The underlying network fault normally shows up as a separate U code in the same or another module. CARB requires digital inputs to be monitored for missing communication. A comparable published check is GM's P06AF in its 2019 tables, where the ECM verifies a message pattern from the transmission module and may stop the engine on a mismatch. For the driver: the engine dies or refuses to restart; check CAN wiring and termination as well as supply and grounds of the modules involved.

Likely causes

Damaged CAN wiring, connector or termination
Partner module without power or ground, or low battery voltage
Failed or wrongly programmed partner module
Software versions of the modules not matching after repair

Symptoms

  • Engine stops while driving or does not start
  • Several U codes and warning messages from other modules
  • MIL or other warning lights on

Repair estimate

DifficultyHardWorkshop job
Parts and labour€0–2,500
Time0.5–6 h

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

Parts involved

can busengine control module

Related codes

P06AFP232AP0600P2105P241D

References: law.cornell.edu, gsi.ext.gm.com, en.wikipedia.org.