Hi, I have an approx 20 yr old lennox gas heated furnace.
I changed the thermostat about 2 weeks ago and now the fan isn't turning ON when the thermostat is in AUTO and HEAT mode. It is working fine in AUTO and COOL.

I switched back to the old thermostat and found out that nothing was working on AUTO. (Furnace not turning ON, blower not running, A/C not turning ON). Since then I have switched back to the new thermostat. Now I have to manually start the heating every time it gets cold.

I did turn off power to the furnace while changing the thermostats.
I would appreciate if someone can put me on the right track to diagnose this issue.

I also wanted to know how can we test the furnace and blower at the same time (connect both W and G to are?)

Thanks in advance!