Back from minding the house while the A/C got serviced:
Me: Could you increase the fan speed?
Nice Young Man: (looks puzzled) Sure, I'll check it out.
Much clomping in the cellar, things turning one and off, etc.
NYM: Have you had the a/c on at all this year?
Me: A couple times.
NYM: According to my diagnostics, the motherboard isn't working.
Me: (in despair) You'll have to give me an estimate.
NYM: I'm going to call my boss to make sure I'm doing this right.
10 minutes later, an older guy arrives. They put new batteries in the thermostat, clomp around the cellar, go outside to check that unit.
Older Man: Did you install a new thermostat?
Me: No. This one is a few years old.
OM: Oh, then it's ours. Well, your wires were crossed. That's why the fan wasn't working. We've fixed that. You should be fine now.
More clomping. I'm sitting at the kitchen counter when the vent at my feet goes WHOOSH and lots of cold air freezes my toes.
The techs come upstairs, I sign the official report, and they leave.
This annual service contract is totally worth it. And honest technicians are a treasure. Whew.