NOx storage catalyst removal

Some direct-injection petrol engines run very lean under light load to save fuel, and
a lean burn makes nitrogen oxides. To deal with that, these cars carry a NOx storage catalyst –
a trap that holds the NOx and then burns it off in short rich pulses. BMW fitted it to the N43
and N53 six-cylinder engines, Audi and Porsche to their FSI and TFSI, Mercedes to the CGI range.

It is not AdBlue. There is no tank, no fluid and no dosing system. It is a catalyst with a
sensor after it, and the sensor is the part that goes wrong.

How it fails

The NOx sensor has a heater and an electronics module of its own, and it is one of the more
expensive sensors on the car. When it fails the engine drops out of lean-burn mode, fuel economy
falls off, and the fault light comes on. The trap itself can also lose capacity with age and
sulphur contamination, at which point the car spends more and more time regenerating it.

The cars this affects are now mostly ten to fifteen years old, and a genuine sensor can cost
more than the car is worth. That is why people ask about the calibration route.

What the calibration does

The engine is told to run without the lean-burn strategy, so it no longer needs the trap or
the sensor to be working, and the fault code and light go away. It gives up a little economy at
light load and keeps everything else as it was.

The legal position, stated plainly

It is an offence in the UK to use a vehicle on the road with its emissions
control equipment removed or defeated. Since 2014 the MOT test includes a visual
check, and a missing filter is an automatic failure. Insurers can and do treat it
as an undeclared modification.

We say this on every page because we would rather lose the job than have a
customer find out at their next MOT. If your car is a road car, the answer is
repair, and we are set up to do that properly.

Where removal is legitimate: vehicles used solely
off the public road, competition vehicles, plant and agricultural machinery, and
vehicles being exported to territories with different requirements. We ask which
applies before we quote.

What we would rather do first

Most of the cars that arrive asking for removal do not need it. They need the
underlying fault finding and fixing, and the system putting back into a working
state. That is diagnostics, and it is a real service rather than a way of hiding a
warning light.

We have the diagnostic equipment and the rolling road to prove a repair worked
rather than assume it did. If a repair is not viable and the vehicle is genuinely
off road, export or competition, we will talk about the alternative.

For the trade

This is one of the jobs available through our dealer file service. Send the
original read, tell us the vehicle and what you need, and the file comes back
checked. If a job carries mechanical conditions on the car, they are listed in the
coverage table below and they apply to your customer’s vehicle too.

More about becoming a JFA tuning dealer