AdBlue system fault: start not possible countdown

Which Mercedes are affected?

Mercedes-Benz diesels that use AdBlue, the fluid the selective catalytic reduction (SCR) system injects into the exhaust to cut nitrogen oxides (NOx). If you are not sure whether your car uses AdBlue, send us the registration and we will check.

DVSA’s emissions recall records include the V-Class built from 2014 to 2019, and C-Class, E-Class, S-Class, CLS, GLC, GLE and GLS built from 2011 to 2019. For other work on these cars, see our Mercedes-Benz specialist page, and for the V-Class our Mercedes V-Class page.

What are the symptoms?

In the owner’s manual for the E-Class, Mercedes describes the sequence:

• the engine warning light comes on with an AdBlue system fault message
• a speed restriction and a distance countdown, shown as “Start not poss. in” followed by a distance
• once the countdown runs out, the engine cannot be restarted

Owners report the warning even with a full AdBlue tank. When you book, tell us the message on the dashboard, the distance left and when it first appeared.

What causes it?

The warning means the system cannot confirm that it is cleaning the exhaust as it should. Known causes include:

• a failed or implausible NOx sensor, often the one after the catalytic converter
• faults in the dosing, the pump, the heater or the level sensing
• poor-quality AdBlue
• a corroded connector or damaged wiring
• out-of-date control unit software
• a faulty EGR valve, which can also lead to SCR faults

How do you confirm the cause before fitting parts?

A fault code is not a repair, and these faults look alike. For one fault profile, Mercedes’ own guidance says that replacing the NOx sensors or the AdBlue tank does not fix it. So before we quote for parts, we:

• check the recall status and the software level first
• read the engine and SCR fault codes with freeze-frame data
• check the connector, supply and wiring at the NOx sensors
• test the AdBlue concentration, line pressure, pump and heater
• compare the NOx readings before and after the catalytic converter on a road test, once the system is up to temperature

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?

We replace only the part the tests prove faulty, using the correct part for your car, then road test again to confirm the system is working and the warning has cleared. If the fault is covered by a recall, the work is done by the manufacturer’s network instead.

We repair AdBlue and emissions systems. We never remove or disable them.

How long does an AdBlue repair take?

It depends on what the tests find. Some checks need the car driven until the exhaust system is up to temperature, 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.

Which recalls should I check?

DVSA lists Mercedes emissions recalls whose remedy is a software update, with NOx sensor replacement if necessary:

• Cars: R/2019/304, R/2019/307, R/2020/231 and R/2020/243.
• V-Class: R/2018/271, R/2018/311, R/2020/241 and R/2020/249.

Whether a recall applies depends on the individual car, so check your registration on gov.uk. Mercedes-Benz UK’s own recall checker also shows non-safety service measures. Recall work is done by the manufacturer’s network, not by us.