[Paradoxfish]

Car is a 2013 335i M-Sport 6-speed with ~88k miles. I've owned it since around 35k miles with regular service, etc.

I just got back from a pretty long trip (about 1400 miles total). The car drove fine, although I did notice slightly reduced gas mileage. I didn't see any temperature issues, but I also wasn't watching the temperature like a hawk.

Anyway, about a day after getting back the car clearly went into limp mode. Feels more or less fine up until 2k RPM, and then power drops like a rock. I've experienced the M-Sport limp mode issue maybe twice in the 5 years I've owned this car, and this feels exactly the same, but turning the car off doesn't clear the problem. The radiator fan is also running at full blast continuously and the temperature gauge is pegged at the bottom, even after a 15-minute drive home.

No check engine light, but I do have two pending codes - P0128 and 1D2008. The codes and the symptoms seem to point pretty definitively to a stuck thermostat, but I'm also finding a ton of posts saying the water pump is a possibility. I'm way the hell over blindly throwing parts at a problem at this point, so I'm looking for a little feedback: should I replace the thermostat alone, or does it make sense to replace the water pump, too? I realize I have to remove the pump to get at the t-stat, but it's also about 4x the cost so it doesn't seem worthwhile if it's probably fine. Was kind of hoping it was just the water pump since the thermostat looks like a pain to reach.

Anyway, I appreciate any input or any advice on anything else that might be to blame.