A Check Engine light system check revealed a possible thermostat problem.
Sounds like you have a code P0128.
The computer monitors the warm up cycle of your engine -- it looks for your coolant temperature to rise to a certain level within a given time frame. The computer does this because it is trying to go into closed loop operation as quickly as possible leading to better fuel economy.
Now the code gets recorded because sometimes the thermostat has become stuck open or it leaks when closed. Either of these options lead to the engine not reaching the desired temperature level.
You would first test that the thermostat is indeed at fault by monitoring a warm up cycle of the engine. If it is, then I'd recommend replacing the thermostat. After replacing the thermostat, record your engines warm up cycle and compare it to before the repair to confirm your issue was resolved.
If you feel like this is a job you can handle be sure you clear the codes and reset the module after performing the repair. Please note that a lot of vehicles have special coolant fill procedures for when you refill your coolant.
If you feel uncomfortable handling this, consider having a certified mechanic, such as one from YourMechanic, come over to handle the repair for you and check if the Check Engine Light is reading any other codes.
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