Thermostat not reaching temperature
P0128 and P0125 mean the coolant did not reach the expected temperature in the time the monitor allows.
The engine may run cool, the heater may blow lukewarm air, and fuel use can rise. A thermostat stuck open is a common reason, but the coolant sensor can set the same codes.
Confirm the code and the warm-up behavior before replacing the thermostat. The DTC page separates sensor checks from the thermostat.
A trouble code marks a condition to test. It does not by itself prove which part to replace.