I have a carrier furnace 2008, just changed my old thermostat to new nest thermostat, forgot to turn off the power. Now I have no power on my r line, checked short fuse still good, red light in front of furnace still on, I am not sure why I lost power to the r line. My furnace was working fine yesterday. Checked with nest supporting line, nest base was not a problem. Help
check for 24 volts at the furnace. Some carrier boards will have a lit led even if the 24 volt circuit is non functioning. I don't recommend using a contact probe. I always recommend using a muti-meter.
Check from R to C. your looking for 24 volts. if no voltage, then check 3/5 amp fuse on board. If fuse is good, check voltage from transformer.
If you do have voltage From R to C. put a jumper from R to G and see if the fan comes on. if fan comes on, then you know the issue is at the thermostat somewhere.
I hope this helps..