CHECK ENGINE LIGHT ON WITH DTC P0126:00 AND ENGINE RUNNING COOLER THAN NORMAL / POOR HEATER PERFORMANCE / LOW ENGINE COOLANT TEMPERATURE (BLUE) LIGHT ON: Some customers may experience the CHECK ENGINE LIGHT ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: The engine takes longer to warm up. The engine temperature gauge fluctuates. The low coolant temperature indicator (blue) light stays on longer or comes on while driving. Poor heater performance, especially at idle.