pressurized 200F water = steam. Steam doesn't leave puddles unless it is a really big leak.
Mine had a crack in the rubber cap on the t-stat housing. It showed no sign of wetness or puddles; steam just dissipated.

 

Anyway, coolant a little low shouldn’t cause a P0171. But if someone sold you fuel with too much ethanol, that might.

 

could if it is going out the tail pipe.

 

🤔

Don't know exactly what the mechanism would be but yeah, that seems plausible.

Or low fuel pressure could cause an excessively lean condition.