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U02A4 Lost Communication With NH3 Sensor

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

An NH3 sensor reports ammonia downstream of the SCR catalyst, showing whether more reductant is dosed than the catalyst can use. Like a NOx sensor it has its own control unit on CAN; Bosch even sells a NOx sensor that delivers a selective ammonia signal alongside NOx and oxygen. U02A4 is set when the engine or dosing controller no longer receives any ammonia messages. No manufacturer documents U02A4 publicly. FCA used a code of its own instead: bulletin 18-028-16, covering Cummins 6.7 L Ram chassis cabs (3500, 4500, 5500) of 2014, names U12A4 for lost contact with the ammonia sensor and offers separate calibrations for trucks with and without it. CARB 1968.2 requires inputs from smart devices that affect emissions to be monitored. Without the signal, dosing runs without slip feedback.

Likely causes

Engine calibration expects an ammonia sensor that is missing or different
Heat damage, water or corrosion at the sensor connector
Fuse, supply or ground of the sensor control unit faulty
NH3 sensor control unit internally failed

Symptoms

  • MIL on
  • SCR system warning
  • Higher reductant consumption possible

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–1,000
Time0.5–3 h

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

Parts involved

nh3 sensorvehicle bus wiring

Related codes

U05A5U029EU059F

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