What it means
The vehicle bus reports loss of communication with the ammonia (NH3) slip sensor downstream of the SCR. Detection of urea over-dosing is impaired.
At a glance
- Usually lights the warning light of the system that lost communication
- Affects emissions; the car can fail an emissions test
How it works
The downstream NH3 sensor measures ammonia slip after the SCR catalyst, helping detect excess DEF dosing. U02A4 sets when the network stops receiving this sensor's messages, so the controller loses feedback that can reveal over-dosing.
Common causes
Most likely first.
- Bus wiring open or shorted
- Module without power or ground
- Module 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
How to diagnose it
- Scan all modules and record freeze-frame data to check whether the NH3 sensor fault occurred with other communication codes or a voltage problem.
- Review live SCR and NH3 sensor status to confirm the downstream sensor's data is absent rather than merely reporting an unusual reading.
- Inspect the NH3 sensor connector and exhaust-area harness for heat damage, corrosion, loose pins, or chafing.
- Check the sensor's power and ground against the manufacturer's wiring information and test conditions.
- Test the network circuit at the NH3 sensor for opens, shorts, and poor connections using the manufacturer's procedure.
- If the network and supplies are sound, verify the NH3 sensor has failed before replacing it.
Common fixes
- Reconnect or repair a loose or corroded NH3 sensor connector.
- Repair heat-damaged or chafed wiring near the downstream exhaust sensor.
- Repair an open or short in the NH3 sensor network circuit.
- Restore the sensor's power or ground connection.
- Replace the NH3 sensor if its supplies and network wiring pass testing.
Can I keep driving?
The vehicle may be driveable, but the SCR system cannot reliably detect ammonia slip and may record emissions faults or warnings. Avoid extended driving if SCR warnings or a countdown appear, since continued dosing without this feedback can worsen emissions-system problems.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.