Codes p0128 and p0171

I have a 2009 Pontiac Wave (Canadian), 56,000 KM.

About a 3 weeks ago the hold light came on (could not turn off using hold button near the shifter) and the car seemed to be shifting "strangely" and rough. Air Cond. stopped working. There was also some hesitation in power. Hooking up a code reader I got a code of P0128 (Engine Coolant Temperature Below Thermostat Regulating Temperature). After erasing the code. The hold light went off and the shifting pattern returned to normal. However the exhaust is a bit louder and she hesitates and bucks at about 40km/h at times. Today the check engine light has came on and now she is reading P0171 (Fuel trim system lean bank 1) Where do I start? Should I replace the thermostat?