oranoco i think the lack of filter changes is part of the problem and some places do tend to just change them with out diagnosing the issue properly,,,,,,, the next stage of wrong diagnosed problems on vehicles is EGR vavles,,,, well sell fucking LOADS of them at work and then get a call back a few weeks later when the light comes back on again

im sure there aint much to them if you take the time how they work ect