I deal with a lot of dpf filters at work
The more you drive the more blocked it will become and the less likely a forced regeneration will be possible, imagine a pipe becoming blocked with cement, more and more being fed down the hole, eventually the hole will get so small and....
Usually If the engine light is on too that means you have gone past the point of doing a regeneration yourself, likely a Nissan technician with a laptop and try to force regen it on the road, 50mph steady speed no stopping.
On the jags atleast if you slow down the regeneration is interupted so not to cause fire.
Remember the dpf clears by pumping in loads more diesel to burn thus heating the exhaust to 600-800 degrees, with no air flow fire would occur..
Not sure with Nissan but again when the light goes out to say it's clear likely it is not empty, just at an "acceptable level" but if you carry on meeting the criteria whilst doing a regen for another 10-15 mins you might find it does not come on. As soon (criteria being no interruptions of speed ect)
Obviously as a town car you will forever be fighting the dpf as in town it will be filling all the time.
Every time you need to regen you will be contaminating the oil as well with diesel requiring service sooner. This occurs when it pumps extra fuel into cylinders to heat exhaust, some.passes the rings.
Hope this helps :thumb2
