Well the 4.8 and the 5.3 are exactly the same motors but the 5.3 has a bigger stroke. I'm pretty sure everything is the same. My mechanic told me that everything matched up. Yeah it was a used motor, had around the same as mine did. Around 130k. They replaced the water pump and thermostat when they did the swap though so i don't think any of those would be the problem.Yes, I'm pretty sure that the coolant temp sensors are the same considering that the block and head are the same things between the two motors. I'm not sure if it was mismatched, my mechanic told me everything would work because they did the same thing with a 5.3 and 6.0. He said he knows someone that worked with GM and was gonna talk to them to see if they know what could be going on.