You really need to have the codes read. Otherwise, you can be chasing anything. I have a Scan II gauge, but even with it I get enough inaccurate readings that I have to rely on the dealership for codes. My current check engine light was caused by a bad glow plug. For a number of years the code was just an occasional anomylous temperature reading for less than 30 seconds. The list can go on.
I'm still on my first dpf, over 500,000 miles.
eb