U05A9NOx Sensor C Received Invalid Data From ECM/PCM
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- With this code the third NOx sensor, fitted after a second SCR stage, reports that it cannot use the frames the engine controller sends it, such as heater release or operating state. No manufacturer documents U05A9 publicly. The closest published logic is Ford's 2023 diesel summary for sensors A and B (U05A1, U05A2): bad input is flagged when a dew point release comes while the key is off or when frames pause longer than 1 s, and the fault is logged after 5 s. GM's 25OBDG04B shows that a sensor C exists, but only for the opposite direction (U05A8). CARB 1968.2 requires communication errors on digital outputs to be detected. If the sensor never gets its heating release, its readings and the second-stage SCR check stay unavailable.
- What you may notice
- Check engine light on
- No NOx value from sensor C on the scan tool
- Likely causes
- PossibleIntermittent bus connection to sensor C
- PossibleEngine controller not configured for the third sensor
- RareSensor C control unit rejecting valid data
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €100 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.