I have a diesel Vauxhall Astra that I've had on-off problems with for a couple of years, mostly DPF related. I've done quite a bit to remedy this including having it taken off and cleaned out and having all the fuel injectors replaced for new ones. This has helped somewhat but the car has always had a tendency to smoke a bit out the back end and also smell. Not quite a typical exhaust smell but an acrid burning smell, so I assumed this was an issue with the DPF.

I've noticed that if the car is travelling slowly or is stationary in traffic AND the heating is on (fan heater is on) the smell gets into the car itself. At first I thought the exhaust might be leaking but I've had several carbon monoxide monitors in the car with me and they always stay at 0 PPM so I would think this rules out an exhaust leak.

There's definitely something there though, and having the blowers on makes it a lot worse, to the extent that if I'm coming to a stop at say some traffic lights I have to turn the heating off until I can get the car moving again else it soon starts to smell inside the car. If this goes on long enough I can feel it irritating the back of my throat, it gives me itchy eyes etc.

I've had a garage look at it and they said they couldn't find a reason, although I suspect they didn't actually look that hard. Does anyone have any ideas? Could it actually be the exhaust even though the carbon monoxide detectors never pick anything up (which I think they would do if it was the exhaust). But if not the exhaust what else could it be?