BMW AdBlue warnings and NOx sensor faults

Which cars and engines are affected?

BMW and MINI diesels that use AdBlue, the fluid the selective catalytic reduction (SCR) system adds to the exhaust to cut nitrogen oxides (NOx). Not every BMW or MINI diesel has it, so if you are not sure whether yours does, put the registration in the enquiry form and we will check.

For other work on these cars, see our BMW and MINI specialist page.

What are the symptoms?

• an AdBlue warning on the dashboard
• a mileage countdown shown with the warning

Do not ignore the countdown. If the AdBlue tank runs empty, the car will not restart.

When you book, tell us the message on the dashboard, the distance left and when it first appeared.

What causes it?

The RAC lists these causes of an AdBlue system fault:

• a failed NOx sensor
• a blocked or faulty AdBlue injector
• a leak in the tank or the pipework
• a frozen system
• a defective pump

Several of these bring up the same warning, so the dashboard message alone does not tell you which part has failed.

How do you confirm the fault before fitting parts?

A fault code is not a repair, and a new NOx sensor will not fix a blocked injector. Before we quote for parts, we:

• read the SCR fault data with our diagnostic equipment
• check the NOx sensor readings
• test the dosing, including the AdBlue injector and the pump
• check the heater and look for leaks in the tank and pipework

We then show you what we found and send a written price. No part is fitted until you give your written approval.

What does the repair involve?

• Replace the failed part, using the correct part for your car.
• Reset the system with our diagnostic equipment once the fault is fixed.

AdBlue and SCR repairs and NOx sensor work are done in-house. Once the work is done, we check that the warning has cleared and verify the repair, including a road test where appropriate. We repair AdBlue and other emissions systems; we never remove or disable them.

Can the AdBlue system be removed or switched off?

Not by us. In the MOT, SCR parts are part of the emissions check, and emissions equipment that is missing, obviously modified or obviously defective is a Major defect. So is an engine warning lamp that shows a fault on a diesel first used from 1 July 2008.

The Department for Transport consulted in 2026 on tighter rules against emissions tampering, naming AdBlue deletion alongside DPF removal.

How long does an AdBlue repair take?

It depends on what the tests find, and some parts may need ordering.

Before any work starts, you get a written price and a promised time for your next update. If the car needs to stay with us, ask about a courtesy car when you book.

Is there a recall for the AdBlue system?

Whether a recall applies depends on the individual car, so check your registration on gov.uk. Recall work is done by BMW’s network, not by us.