Electrical and Controls
Ohm's law and the 24-volt control circuit
Nearly every no-heat or no-cool call begins at the transformer. Ohm's law (E = I × R) lets you predict current through a contactor coil or gas valve and spot a transformer that is being overloaded. Control circuits are series circuits: the thermostat, high-limit, rollout switch, pressure switches, and float switch all sit in series, so a single open safety kills the whole circuit. Learn to trace a ladder diagram from the transformer's secondary through each switch to the load, and note which switches are drawn normally closed.
Motors, capacitors, and relays
PSC motors use a run capacitor sized in microfarads; discharge it before testing and test it out of circuit. A failed run capacitor produces locked-rotor amperage and a tripped overload. On compressor terminals, the highest resistance reading is start-to-run, and that should equal the sum of common-to-start plus common-to-run. ECM and variable-speed motors take a 24-volt control signal but carry high-voltage DC internally - never ohm a live board.
Traps
Assuming an open safety is the failed part when the real fault is upstream; testing a capacitor in circuit; trusting a digital meter that is reading ghost voltage on an unloaded wire; overlooking a missing C wire on a smart thermostat; and misreading a contactor with pitted, welded points as "good because it's pulled in."
Rule of thumb
A run capacitor that measures more than about 10 percent below its rated microfarads should be replaced, and it is the first thing to check on any single-phase motor that hums and trips its overload.
Worked example
A 40 VA transformer feeds a 24-volt contactor coil measuring 12 ohms. Current = 24 ÷ 12 = 2 A, so the coil draws 48 VA - more than the transformer can supply. The transformer will overheat and the coil will chatter.