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U05A5 Invalid Data Received From NH3 Sensor

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

An ammonia (NH3) sensor behind the SCR catalyst tells the controller whether too much urea is dosed and ammonia slips through. Like NOx sensors it carries its own electronics and talks over CAN; Bosch even offers NOx sensors that add a selective ammonia signal. U05A5 means the ammonia data come in but fail validation. FCA's bulletin 18-028-16 for 2014 Ram 3500 to 5500 Cab Chassis with the 6.7 L Cummins shows a special case: two ECM calibrations exist, with and without the ammonia sensor, and if U05A5 or U12A4 appears after the update, the wrong one was loaded. CARB 1968.2 requires a monitoring plan for such other exhaust gas sensors. U02A4 means the sensor is silent. Dosing control loses its slip feedback.

Likely causes

Engine calibration does not match the sensor fit (with or without NH3 sensor)
Ammonia sensor control unit faulty
Damaged wiring or connector at the ammonia sensor

Symptoms

  • Check engine light on
  • Code appears right after an engine software update
  • SCR dosing runs without ammonia feedback

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–900
Time0.3–2 h

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

Parts involved

nh3 sensor

Related codes

U02A4U059F

References: static.nhtsa.gov, bosch-mobility.com, law.cornell.edu.