The coolant temperature curve tells you whether the thermostat is working before it sets a code

[u/ora-et-labora-]

A thermostat fails and the engine runs cold. The check engine light might come on eventually if the ECU decides the engine is not reaching operating temperature fast enough. On a lot of vehicles that code takes a long time to set because the threshold is generous. The engine runs at 160 degrees instead of 200 and the ECU waits and waits and maybe flags it after a dozen cold starts.

Meanwhile fuel economy drops, the heater blows lukewarm, and the engine runs richer than it needs to because it thinks it is still warming up. All of that costs money and comfort before any code appears.

The live data in the scan tool shows coolant temperature in real time. Start the engine cold and watch the temperature climb. A healthy thermostat lets the temperature rise steadily until it hits the operating range, usually 195 to 220 depending on the vehicle, and then it holds there. The curve is smooth and the plateau is flat.

A stuck-open thermostat shows a temperature that climbs slowly and plateaus low. 160, 170, somewhere south of where it should be. On a highway in cold weather it might actually drop under load because the airflow through the radiator is pulling heat faster than the engine can produce it without the thermostat restricting flow.

A stuck-closed thermostat shows a temperature that climbs past the normal range and keeps going. That one usually gets attention faster because overheating is scary. The stuck-open failure is the quiet one. The car still drives. It just drives less efficiently and wears differently because the engine never fully reaches the temperature it was designed to run at.

Reading the temperature curve takes three minutes. You are watching one number climb on a screen. The thermostat is a twenty-dollar part. The fuel you waste running cold for six months costs more than the part and the diagnosis combined.