Thermostat problem on VUE

Last Friday I brought my 2002 V6 Vue to the dealership for a SES light
on. They checked it out and found code P0128 on. This is the Cooler
Thermostat (Cooler temp below thermostat regulating temperature). It is
a 2 hour job to replace it so I made an appointment to have it repaired
the next Friday. They told me there was nothing wrong with the
thermostat. It was giving off a false reading. They also told me that
the SES light would probably come back on before Friday.
The next day I was driving down the highway and noticed that I had
reduced power when I tried to accelerate (The reduced power light isn't
on) The acceleration on the vehicle is just passible. OK for the city
and getting on the highway at a reduced time but if I had to pass forget
it.
My question is if the SES turns on for a thermostat problem will the
vehicle be working at a reduced level or do you think something else
might be wrong. Any feedback would be greatly appreciated.
Ron

 

If the computer is getting bad temperature information it is entirely
plausible that it would adjust timing, fuel mix and everything else
incorrectly based on the bad data. This could easily result in a poorly
running engine. It sounds more like the temperature sensor is bad not
the thermostat but either way I'd make sure they fix it.