I have been having problems with my 03 cavy overheating. Highway it was fine... city driving it overheats. I diagnosed my cooling fan yesterday, and it was not kickin on. A new relay fixed that.

I left the store, was driving through the city without the AC, and all was well. Once I hit the highway, I put the AC on and left it on.

Now today, at idle it starts to overheat. If I turn on the AC, the cooling fan kicks on, and it brings the temp back down.

What should I check next? Im thinking my possible options are:
-Bad thermostat (but wouldnt it overheat on the highway too?)
-Bad coolant temp sensor (But there is no CEL)
-Air in the cooling system?
-Relay going bad again?

Unsure of where to start... Im thinking of replacing the thermostat... and if that doesnt work, maybe changing the coolant temp sensor next. Thanks!

Which engine you running.? since idk I'll assume yours is like mine for this but anywho..

How do you know it's over-heating? Is the dash needle in the red? or just near? If the needle is responding like that, the sensor is good.
If your in the red and the fan doesn't come on check fuses/ relays. At idle it should get pretty close to red before the fans come on, and it should drop back down after a couple mins at idle after the fans start running with the AC off.

With my OBDII bluetooth adapter I can read my coolant temp digitally on my phone using the torque app. I think the fans come on around 205°F