Audi A4 AdBlue Warning & NOx Sensor Fault — ODIS Diagnosis

CUSTOMER COMPLAINT

The driver was greeted by an AdBlue warning and the engine management light, with a message about a restricted range and the prospect of a no-start lockout if ignored.

SYMPTOMS

AdBlue/DEF warning on the dash, the engine management (MIL) light, and a countdown warning that starting would be disabled after a set distance.

DIAGNOSTIC PROCESS

Connected ODIS for a full scan of the engine and emissions modules. The fault codes pointed to the NOx sensor circuit on the SCR system. We ran the ODIS test plan for the NOx sensor, checking live data and wiring before condemning the sensor.

FAULT FOUND

The upstream NOx sensor was faulty, returning implausible readings and triggering the AdBlue/SCR fault — and with it the engine management light and the range countdown.

REPAIR / SOLUTION

The faulty NOx sensor was replaced with a genuine part, the AdBlue dosing system was reset and primed, and the fault codes cleared. The car was then road-tested under the conditions the SCR system monitors to confirm the NOx readings were back in range and the dosing correct.

RESULT

All warnings cleared and the range countdown disappeared. Road-test data confirmed the SCR system working correctly, so the customer could drive without the threat of a no-start lockout.