U02A4Lost Communication With NH3 Sensor
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- MIL on
- SCR system warning
- Higher reductant consumption possible
- Likely causes
- PossibleEngine calibration expects an ammonia sensor that is missing or different
- PossibleHeat damage, water or corrosion at the sensor connector
- PossibleFuse, supply or ground of the sensor control unit faulty
- RareNH3 sensor control unit internally failed
- 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: €50 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.