P0128 and P0420 on 2001 Volvo S40

So i’ve had this volvo for about 6 months now.
I knew when I purchased it that the two codes being thrown were p0128-Engine coolant temperature, and p0420- Cat efficiency Bank 1.
Inspecting mechanic said it needed a new cat to clear the p0420…
I went ahead replaced the thermostat and changed the coolant to “fix” the p0128, and cleared the codes, I think the jiffy lube state inspector did not notice the that the catalytic converter test was Incomplete when he passed it. Lucky me I guess.
But now I know that i’ll probably have to fix before it gets inspected again. Eventually the p0128 came back on, so I went and got a new ECT sensor. Still doesn’t clear the p0128. What am i missing? Coolant temperature sits at a 3’oclock position pretty regularly and quickly.
I have no idea where to start on the p0420 besides understanding its the Pre-cat o2 sensor that is throwing the code, is there anyway that these are related?
Thanks for your help!

Whatever you do, do NOT let someone talk you into replacing the cat before checking other possible culprits for the P0420 code. That code can be produced by a bad downstream (not upstream) O2 sensor that misreads O2 downstream of the cat and therefore mistakenly reports low cat efficiency. Have that sensor pulled and tested by someone who knows how.

Just replace the rear oxygen sensor…I would ignore the P0128 as long as your temperature gauge looks okay…Is your radiator full to the brim when the engine is cold?

Did you use a thermostat of the same temp as the original, OEM preferably?
It’s conceivable the cat isn’t working right because (the computer thinks) the engine isn’t properly warmed up; upsetting the fuel air ratio.
The general rule is to deal with the lower numbered code(s) first.