My engine control light is on in the fall of 2009, so I removed the code. This was P0128 – Coolant Thermostat (coolant temperature lower than the thermostat control temperature). I decided to check and replace the thermostat. That was the problem and it was fine since. Overall, the job took about 45 minutes to 1 hour, including emptying, warming the engine and refilling the cooling system. It was a pretty easy solution. This problem causes the engine to turn too cold because the thermostat is stuck in the open position, causing more coolant to flow into the radiator than necessary. Ignoring this problem causes the fuel mixture to be too rich, resulting in poor fuel economy.
Video credits to JGonYT YouTube channel