Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Thermostat Monitoring: Description
The diagnostic checks for a partially open thermostat, under conditions when the thermostat would be expected to be shut. See Fig 1 .
A second ECT sensor is installed in the outlet from the radiator. If the enablement criteria are met and the ECT is less than the normal thermostat opening temperature the diagnostic will run. See Fig 2 .
The diagnostic compares the difference between ECT and the radiator outlet temperature. This gives the temperature drop across the radiator. See Fig 3 and Fig 4 .
If the temperature drop is less than a threshold, and there is flow across the radiator, this is caused by leakage through the thermostat.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.