Hello All
I have two separate air conditioning units and two thermostats for 2 zones. The Air conditioner is from Trane and about 10yrs old. I have a problem on one of the units/zone.

When the thermostat is set to heat mode - and the heater turns on - The outside air conditioner unit fan also turns ON. Don't notice any issues inside. Hot air is blowing from the vents just fine. I don't experience this with the other zone and air conditioner unit. I also replaced the thermostat unit inside the house. The wiring to the thermostat is Red, Yellow, White, Green Wires.

No issues in summer - both air conditioners work just fine...

Any experts may know what the cause may be?

Thanks for all your help