AdBlue countdown or no start: what to do now

Cars and vans that use AdBlue are built to type-approval rules, still part of UK law, which say the vehicle must not keep running without working NOx control. The rules call this a driver inducement system. The countdown is deliberate.

There are two ways in. The first is level: after an earlier low AdBlue warning, the countdown starts once the AdBlue left would only last around a tank of fuel's driving, or less, depending on the make.

The second is a fault. If the system sees fluid that is not to specification, dosing that has stopped, AdBlue use well off what the engine asks for, or too much NOx at the sensors, it warns you first. If it is not put right within 50 km (about 30 miles) of that warning, the countdown starts, set at roughly the distance the vehicle covers on a full tank of fuel.

Some vehicles count down engine starts rather than miles. Others also restrict speed, to no more than 50 km/h (about 30 mph) shortly before restarts are blocked. Heavier vans approved to the heavy-duty standard follow separate rules.

Wording varies by make, model and year. You might see "Starting impossible in X miles", "No start in X miles", "No engine start in X km" or "Engine start not possible". Volkswagen pairs its AdBlue level lamp with "No engine start in XXX km". Mercedes-Benz uses "No start in" with a distance, sometimes alongside "Check AdBlue".

The rules expect the warning to give the reason, such as a low level, incorrect AdBlue or a dosing fault, so read the whole message, not just the number. If it says top up or refill, fluid is the first step. If it mentions a fault or the emissions system, more AdBlue will not clear it. Our AdBlue warning light guide covers the message types in more detail.

The miles left are your window to sort it on your terms.

If the count is in its last few miles, or the van has dropped into a speed-limited mode, treat it as a breakdown. Leave the motorway or fast road at the next exit or service area, and only switch off somewhere you can safely leave the vehicle.

Do not stop on a hard shoulder or verge just to check something. It may not restart, and the Highway Code warns that hard shoulders give less protection than other places to stop.

If it has already stopped somewhere unsafe, follow Highway Code rules 275 to 277: hazard lights on, get out on the side away from traffic if it is safe to, and wait well away from the vehicle, behind a barrier where there is one. If you are stuck in a live lane and cannot get out safely, stay in with your seat belt on and call 999.

Where the system blocks restarts, it does not stop an engine that is already running. The block applies the next time you switch off.

If the tank ran dry, add at least the quantity your handbook gives. The rules let the system stay locked until a set minimum has gone in, and that minimum differs between makes. Park level if you can and follow the handbook's restart steps, as some vehicles need the ignition on for a short time before they read the new level.

If a fault caused it, fluid will not help, and repeated attempts to start can flatten the battery. The fault has to be repaired where the vehicle is, or the vehicle moved to a workshop. In Devon, our own technicians attend. Outside Devon, we book a vetted independent technician near you where we have one, and we'll tell you on the call whether we can get someone to you.

The rules say the restriction must not switch itself off until its cause has been put right. A reset is allowed after a repair, so the vehicle can run up to 50 km (about 30 miles) to confirm it. If the fault is still there, the full restriction comes back.

The vehicle also keeps a record. The reason for each countdown and the distance driven with it active are stored where a code reader cannot erase them, for at least 800 days or 30,000 km. You may also see a code from the P2BA7 to P2BAD group. SAE J2012 titles these "NOx Exceedence" codes by cause, such as "Empty Reagent Tank" or "Insufficient Reagent Quality". They tell you why, not which part failed.

Clearing codes also wipes freeze-frame data, one of the best clues to the real fault. So we do not reset a countdown without repairing the fault. The reset is the last step of the repair, followed by a check that the system is working.

These let us give you a straight answer first time.