Thermostat shuts off when fan comes on

So whenever my fan comes on my thermostat will go from 210 down to cold again and then the fan will shut off and it slowly goes back to 210 and if I’m parked or wheeling going slowly it will keep doing this over and over.

Any ideas? Whenever it does that I also get the check engine light for thermostat low voltage

Not sure if I am missing something but this sounds normal to me. Your fan should kick on when the temp rises (210 sounds about right) and then shut off once it has cooled the engine off. If it is doing this frequently then you could have a higher mixture of water to coolant which should be around a 50/50 mixture. You can test this with a cheap hydrometer found at Autozone etc… or you could be a little low on coolant.

This sounds like a corroded connection either at the thermostat or fan. Disconnect the connectors and look at the terminals for corrosion. clean/replace as needed. Also check your ground to see if it is loose and/or corroded.