* Incorrect Coolant Temperature Readings: The thermostat regulates coolant flow to the engine. A faulty thermostat (stuck open or closed) can lead to inaccurate coolant temperature readings being sent to the Engine Control Module (ECM). The ECM relies on this temperature data to control various engine functions.
* ECM Detects Abnormal Conditions: If the ECM detects the coolant temperature is consistently too high or too low, it can interpret this as a problem and illuminate the "Service Engine Soon" light to alert the driver. This is because incorrect temperature can lead to inefficient combustion, potential overheating, or other issues.
* Other related problems: A stuck closed thermostat can cause overheating which can damage other engine components, leading to further diagnostic trouble codes and the SES light.
In short, a faulty thermostat isn't the *direct* cause of the light, but it can cause conditions that *trigger* the ECM to illuminate the "Service Engine Soon" light. You'll need to use an OBD-II scanner to retrieve the specific diagnostic trouble code (DTC) to determine the exact reason for the light. The DTC might point to the thermostat or a related issue caused by the thermostat malfunction.
