I had a useful one that's an easy fix (if that's the only thing that wrong!). As things are heating up outside, I happened to be in the basement when our ground-source heat pump unit kicked on. Instead of the familiar click and hum of the compressor starting, I heard a click and wub, no hum. Thinking, "hmm that's not good" I went over to the unit, which didn't show any errors, and shut it off on the control panel then turned off the local disconnect. After waiting about 5 minutes for any energy to dissipate (capacitors), I removed the lid.
I wasn't familiar with the symptom, but knew enough about the motor and compressor setup to understand there was a problem someplace. A quick online search brought up some similar issues in a WaterFurnace chat forum that the compressor motor run capacitor could be bad. If you don't know what that is, it looks sort of like a V-8 juice can, right circular cylinder, that is connected with large wires into the motor/compressor circuit. Here's the control panel side view of it (with tan and blue wires attached).
Usually when they go bad, they short and the can overheats and puffs out on the ends. This one was very slightly domed, but when I disconnected the spade terminals and took it out, it tested open with an Ohm meter, and no reading on a capacitance meter (the can said 370VAC and 80uF or microFarad).
The chat forum had also mentioned the manufacturer went from a 440VAC rated capacitor down to a 370VAC for home units, which is normally fine since they run 240VAC. We had recently had a power surge that took out a bunch of stuff in the house (that the power company paid to have fixed!) and could have damaged this too. I was able to find this capacitor (440VAC, 80uF) at my local Grainger for $53, in stock. I drove down to get it and the tech there also verified with their capacitor tester that the old one was open.
Came home, popped in the new one after removing the adapter plate which had a 2" opening instead of 2.5" for the larger 440VAC rated cap, closed it up, turned it on.... and the compressor started!
While I was in there however, I also noticed that there was quite a bit of dust underneath the compressor start contactor, and that the control board looked a little cooked under a transistor and power resistor. Rather than get that far into it, I called my local service guys that installed the unit (HEY, EVEN I HAVE LIMITS), and they came out and replaced the control board and contactor FOR FREE UNDER WARRANTY. I was 8 years into a 10 year warranty. Nice. The contactor was severely pitted from cycling for 8 years, plus cycling for at least several days with a locked-rotor compressor. They also checked out the unit and topped up the outside loop pressure, and verified it was delivering about 63,000BTU for a 5 ton (60,000BTU) rating. All good.
Technical note: For those of you not familiar with large single-phase motors, they typically have capacitor in series with one motor winding to create a "fake" second phase that makes the motor run much more efficiently. With that capacitor open, the compressor was trying to turn two windings with only one connected, and that no worky. See the compressor and cap in the schematic below:
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