Electrician Service Call, Troubleshooting a 24-Volt Thermostat for Heat in someone’s Home, Thoughts?

My name is Hendy. I'm a 25-year-old electrician on emergency service call. Let's get right to it, guys. So, today a customer called and said that they weren't getting heat in their house and that their thermostat went bad since it wasn't showing any more display even when they tried to change the batteries. So, it's time to troubleshoot. So, the first thing I do is I go to the electrical panel and I check to see if all the circuit breakers are on and there are no trips. Then, I check the furnace to make sure it's on and it was. So, then I go upstairs, which leaves me with a thermostat. I start taking out the old thermostat and put in the new thermostat. Thermostat wire is typically 18 gauge. Tiny wire, not big enough to get zapped. So, I'm working live. I'm not saying to work live. You should never work live. Only when you have to schedule a shutdown if you need to. So, a thermostat is usually 24 volts. The red wire is the main power wire, which always goes on the R terminal of the thermostat. The call for heat wire goes on the W terminal of the thermostat, and the call for cool wire goes on the Y terminal of the thermostat. It may sound a little bit confusing, but it's not rocket science. I tell people in the field, you just got to do it to know. In electrical, you're always learning. Boom. New thermostat is up. The heat is working. Customer is satisfied. I packed my tools.