Thermostat on Hybrid

I'm wondering if there is a good way to tell (besides pulling it) if a thermostat might be stuck open on a hybrid. The problem is that the engine does not run as often so I'm not sure how to interpret the data.

Here is an example of the mpg table as the car is driven for about a half hour in very cold temp, around 15* f.



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I would go old school measure the temperature at raddiator top hose inlet with the by-pass (heater) hose temperature close to the cylinder head as the engine warms up from cold. You should be able to see a huge difference between the two until thermostat opening temperature is reached. I use a 2 channel K-Type thermocouple for this as they react faster than other thermometers. 2 channel thermocouple meters are very cheap on eBay and Amazon

Link www.westernthomsonindia.com/product_info.php