As has been said, being able to pass an MOT doesn't mean a car is legal to drive on the roads. It just means it meets a minimum standard, mainly for safety reasons.
What I'd be interested to know is, do the construction and use regulations say that a car has to have the emission control equipment it was built with or the emission control equipment the legislation required at the time it was built.
There's a difference. Euro V required emission control equipment that effectively means diesel cars have to have a dpf (although it doesn't specifically ask for a dpf but it has to meet particulate levels, which in practice means it needs a dpf of some sort). However, lots of cars had dpf's before Euro V became law. So, do you only need to keep the DPF if it was built/registered after Euro V became law or do you have to keep it even if Euro V wasn't law at the point it was built?
Edit - perhaps it's worth looking on the log book to see what Euro emissions standard it has for your cars. If it's EU5 then you probably shouldn't remove the dpf (I'm not an expert on EU law though).
What's the risk? I guess ultimately, if you have an accident and you've removed the DPF when you shouldn't have then you're technically driving a car that was illegal to use on the highway. Would that invalidate your insurance? Would you be prosecuted?
Last edited by smarty156; 25-11-13 at 12:54.