So after driving a few days with the lamba light off, it came back on and the same code is back P0116 and the fan runs all the time even when car is not hot....so I think it is time for a new thermostat and coolant temp sensor. But it's also due for a timing belt and tensioner, etc so here is my list of parts to order:

1. Timing belt (from IPD with DVD step by step instructions)
2. Cam lock took from IPD
3. tensioner for timing belt (this is the 2nd change and not done 1st time)
4. Water pump (on the old 850 that started leaking around 200k and I am told it's easiest to do along with timing belt)
5. thermostat (possibly the "80 degree" one if I can figure out what that means
6. coolant temp sensor

Eek this is scary so many things to do at once

Is there anything else I should add while I am at it? Other sensors I am missing or parts that I "may as well do while I am in there"? Fuel filter (for some reason that one scares me most I hate messing with gas)?

What about brake parts what's the lifespan there?