getting an engine light for insufficient coolant temp for closed loop, changed thermostat and changed one of the temp sensors but light came on again after a mile or so, I'm wondering if I changed the wrong sensor
My car has 135000 miles.
My car has an automatic transmission.
The answer to your question is - both. The ECU factors in temperature readings from both thermostats to factor in the operating temperature of the engine. In most cases, when you have a fault like this, an OBD-II trouble code will be stored in the ECU that will indicate which sensor is faulty. I would recommend replacing the other sensor, but having a professional mobile mechanic come to your location and complete a check engine light inspection so they can clear the codes.
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