OBD-II code · Network
U02A4 Lost Communication With NH3 Sensor
The vehicle bus reports loss of communication with the ammonia (NH3) slip sensor downstream of the SCR. Detection of urea over-dosing is impaired.
Is it safe to drive with U02A4?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyBus wiring open or shorted
- PossibleModule without power or ground
- PossibleModule failed
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–3 h
- Emissions test
- Likely to fail
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Nh3 Sensor, Vehicle Bus Wiring
Related codes
- U02A1Lost Communication With NOx Sensor C
- U02A2Lost Communication With Reductant Quality Module
- U02A3Lost Communication With Particulate Matter Sensor
- U02A5Lost Communication With Reductant Heater Control Module
- U02A6Lost Communication With Safety Integration Control Module
- U02A7Lost Communication With Engine Sound Control 1
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.