Mechanic Q: truck runs cold all the time, is thermostat stuck open??

I had head gasket leak, I put in sealer which sealed leak, but also sealed up radiator. I had radiator shop put in new radiator. The truck now is slow to warm up and runs cold on descents, normal on level and uphills. Did the radiator shop put in the right size radiator, since there two, one for manual and one for automatic, or is the thermostat stuck open?

A: truck runs cold all the time, is thermostat stuck open??

I put new thermostat in and the problem is solved, thankyou