Recently started looking at the DPF regen logs for my car (2009 35d) and had some mild concern that it hadn't happened for quite some time. Looking deeper into the problem, there were a couple of clues - all glow plugs and controller replaced a few months ago (all failed/weak) and engine temp never getting above 70 degrees.
So back to the dealer...
Me: The car isn't getting up to temperature. It won't go above 70 degrees and the optimum operating range is around 88 degrees.
Them: Certainly Sir, we'll look into it.
4 hours later
Them: Car is all ready.
Me: Great - did you have to change the thermostat.
Them: No, we turned the heater system up
Me: Why?
Them: You said the car wasn't getting up to temperature, so we turned the internal heater up.
Me: The engine temperature! Not the cabin temperature. Why would I want the cabin temperature to be 88 degrees?
Them: Oh

1 day later
Them: Car all checked and it is fine.
Me: Great - what is the engine temperature now?
Them: Its within operating parameters
Me: What are the parameters?
Them: I can't tell you but the mechanic said they couldn't find any recorded faults
Me: Give me back my car and don't touch anything else! >:(
So, without any recorded faults, there is no problem to fix and the dealer won't do anything. Given the main thermostat is a PITA to change and there is no EGR one, I'll probably go to an indy to get it changed.
The question though is when I do this, and find a fault with the thermostat, could I claim it under the extended warranty?