I just had my thermostat housing and sensor.

I just had my thermostat housing and sensor replaced at Pep Boys and my coolant temp gauge will start out in the middle then jump to almost red then go to 3/4 then back to almost red. No check engine light. I took it back to Pep Boys and they said it's probably a bad gauge because the coolant temp reads at normal temp. Before this happened my coolant temp gauge would stay in the middle after starting my car and stay there during regualr driving. When this issue arose the check engine light came on but the coolant temp gauge stayed in the middle. Is it a faulty gauge or is the new thermostat and sensor that Pep Boys replaced bad? Any help will be greatly appreciated!