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Howdy I got this code on my 05 Elise and was wondering how I tell if it’s my temp sensor or my thermostat. I suspect it’s my temp sensor as it seems like it’s been acting a bit squirrelly.
 

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i think there are other specific codes for the sensor itself when it fails open circuit or short circuit. as long as the wire doesn't look exposed in a way that would cause intermediate voltages, i think the sensor isn't suspect.

From the service notes:
Code:
Coolant Thermostat P0128
P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Description
The thermostat diagnostic is enabled after each cold engine start, and monitors the rate of temperature rise 
during warm up relative to the measured engine air flow.
Monitor: Continuous
Enable Criteria:
•	 Engine running
•	 Coolant Temperature > -10°C (14°F)
•	 Coolant Temperature < 70°C (158°F)
Disable Criteria: 
P116, P117, P118 – Engine Coolant Temperature sensor faults
Potential failure modes:
Thermostat failure
Notes:
The MIL will be illuminated if the fault is present for 2 consecutive trips.
[\code]
 
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