Overheating when the A/C is on

This could be the thermostat and it could be a few other things. Namely, a cooling fan issue or a restricted system from another issue like a plugged radiator.

Basically what is happening is the A/C is adding a larger load to the engine and if the system is restricted it can't handle that added load.

To start, I would take a laser temperature gauge and check around the engine and cooling system for any hot spots. The hot spot will be due to the restriction. We need to check the radiator as well.

Then make sure the fans come on like they should.

If all this checks out then we can get into a possible sensor issue. However, that is less common.

Here is a guide that helps with this:

https://www.2carpros.com/articles/symptoms-of-an-overheating-engine

Let us know what you find and we can go from there. Thanks