What causes a DPF to fail?

Hello, this is getting into very specialized diagnostic testing of an advanced Diesel emissions system -- which requires specific know-how for this particular engine and emissions system as well as accurate data available and being analyzed. Without looking at all the information, it's going to be impossible to know. I will say that I don't see how replacing an engine coolant temperature sensor can affect a P0420 code. A DPF can reach the end of its useful life and require replacement, but it's premature to expect this to be the case. I'd suggest perhaps a second opinion. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops