What valve do you have?

The motor thermistor is indicating over temperature, If this is a older unit and just started to error, check your valve adjustment. They need to run into the floor quick. Check your motor for overheat. If it's wye-delta, time for a quick transfer and check contacts and motor connections. If the valve and motor is fine, and you have high traffic, this is a sales opportunity for a cooler. We installed many oil coolers on the holeless units.

Is the oil actually hot? Did you temp it? Jump out the thermistor if it's not over 130-140. Have had many bad motor thermistors over the years

Bad motor thermistor! (Thanks! 2 years late on the reply and thank you !)

Old thread I know but question on the same issue. Is the thermistor something that can be replaced or is an oil cooler the only option left? If it can be replaced, where can I get the location and part number. Cannot find it in the catalogs.

It seems like they're talking about, what I've heard refered to as, the motor thermal cut out or thermal overload. It that self resetting bimetallic switch inside the motor that opens when gets to hot and closes when it cools down. Those should just be two 18 gaugeish wires coming up from the motor with the motor leads going into the controller and usually labeled P1 & P2 from the motor. You can jump them out or get an adjustable thermistor and wire it in place of the one from the motor so you still have protection

I don't think you can replace the one in the motor with out pulling it and possibly splitting the motor apart. But I don't see why you couldn't replace it with an external one