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U05A9 NOx Sensor C Received Invalid Data From ECM/PCM

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modePossible

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.

Likely causes

Intermittent bus connection to sensor C

Engine controller not configured for the third sensor

Sensor C control unit rejecting valid data

Symptoms

Check engine light on
No NOx value from sensor C on the scan tool

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€100–900

Time0.5–2 h

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

Parts involved

nox sensor cecm

Related codes

U05A8 U05A1 U05A2 U02A1

References: fordservicecontent.com, gsi.ext.gm.com, law.cornell.edu.