Thermostats can get stuck due to normal wear over time. My guess is that the thermostat in your 2000 Jeep Grand Cherokee is stuck open causing the check engine light to come on. A stuck open t/stat will cause the engine to run cooler than designed resulting in an increase in emissions, Symptoms would include a temperature gauge reading lower than normal and a check engine light on. A stuck closed t/stat would cause the engine to overheat - very quickly.
Get the code for the Check Engine Light, and be sure that you radiator is full and that your heater control valve is working to allow hot coolant into your heater core. If that is all ok and your engine is not getting hot enough, then replace the thermostat.