U02A4 code meaning
Lost Communication With NH3 Sensor
Network system Generic code U02xx codes Check engine light
What U02A4 means
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.
U02A4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of U02A4
- MIL on
- SCR system warning
- Higher reductant consumption possible
Common causes of U02A4
- 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
- Less likelyNH3 sensor control unit internally failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nh3 sensorVehicle bus wiring
What each part of U02A4 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| A4 | The exact fault in that area. |
What to do next
- Note U02A4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U02A4
What does U02A4 mean?
U02A4 means "Lost Communication With NH3 Sensor". An NH3 sensor reports ammonia downstream of the SCR catalyst, showing whether more reductant is dosed than the catalyst can use. It is a generic code, so it means the same on every make.
Is it safe to drive with U02A4?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will U02A4 fail an emissions test?
Yes, it can. U02A4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U02A4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U02A4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.