I bought my 325d 2007 with unnervingly clean exhaust tips. And shortly after having DPF warning light comes on, 1k later still on and even after thrashing for miles on the motorway, in what I have read was ideal conditions. Checking engine temp was above 75c etc --- no joke. However, after extensive research I successfully enabled the car to perform a regen, after 20 miles.
Car will not perform re-gene if ...
1. Engine temp does not reach over 75c, and stay this for a period (20 or 30 minutes).
2. Some fault codes are stored, mainly and ironically the DPF fault code itself.
3. Over 20 Litres of fuel in tank (I half filled the tank for good measure).
How I got my car to regen.
First of all, I install BMW INPA (I bought on aobd2shop) on my PC as that I used on my old E46 --- (any fault code scanner will do I think). I read fault codes and cleared them. Within a few miles drive I plugged in and checked again, they were back. I guessed these codes where stopping the car regenerating and it was stuck in a loop.
1. I engaged the engine temp in hidden OBC so I can see temp.
2. Plugged in PC to car (battery fully charged)
3. Went for drive on M5.
4. Engage Cruise Control --- Adjust car to over 50 mph so engine temp runs at over 75 (my temp was reading 76 to 79)
5. Look at your OBC temp reading.
6. After 10 miles my temp started rising and peeked at 93c, this is DPF doing its thing.
7. Rear view mirror = sooty clouds. Occupied by eggy smell later.
8. Exhaust tips sooty.
I suspect the EGR thermostat was the initial culprit but once it throws the DPF warning fault codes keep the car stuck a loop. I noticed the car was storing the DPF code before it was reporting it on the dash when I was driving too. Hence the need to keep clearing codes obsessively when driving.
Hope this helps you out.