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P23A9

Communication Fault Forced Engine Shutdown

A safety-critical communication loss between control modules has triggered a forced engine shutdown. The ECU could not validate signals it needs to operate safely (e.g. throttle commands, brake state) and stopped the engine.

  • Check engine light: Yes
  • Emissions related: No
  • Repair difficulty: hard

Common causes

Ordered by how often each turns out to be the cause.

  • Bus communication lostCommon
  • Gateway module routing CAN messages incorrectlyLess likely
  • CAN terminating resistor (120 Ω) drifted out of specLess likely

Parts involved

  • Powertrain Bus
  • ECM

P23A9 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.