What you'll see
You set the thermostat to cool, drop the setpoint, and nothing happens. No blower, no condenser, no click. Dead quiet. Before you assume the worst, remember that a totally dead system is usually a power or safety problem, not a failed compressor — failed compressors usually still hum or trip a breaker. Dead-silent means something upstream cut the chain.
Work from the wall power down to the low-voltage controls, and stop at the first thing that's missing what it should have.
Walk it in order
- Confirm the thermostat is actually calling. A blank screen is its own branch (dead stat / no 24V — see the blank-thermostat article). If the screen is lit and set to cool with the setpoint below room temp, it should be commanding the system. Bump the fan to "on" — if the blower won't even run on a straight fan call, the problem is power or the blower circuit, not cooling.
- Check line voltage at both pieces of equipment. Breaker for the air handler/furnace, breaker for the condenser, and the outdoor disconnect. A tripped condenser breaker that won't reset, or resets and trips again, is a line-voltage fault (see the tripping-breaker article). No power to the air handler kills everything because the transformer lives there.
- Check the low-voltage fuse and the transformer. Most furnaces and air handlers have a 3A or 5A blade fuse on the control board. Blown low-voltage fuse = something shorted R to C — find the short, don't just keep replacing fuses. Measure 24V across R and C at the board. No 24V with good line voltage and a good fuse points at the transformer.
- Check the safety switches in the chain. A float/overflow switch on a clogged condensate line is the single most common "everything's dead" cause people overlook — it opens the 24V and the whole system goes silent. Check the float, the pan, and any auxiliary drain-pan switch. Also check the blower-door safety switch; a panel that isn't seated kills the system.
- Once you have 24V at the board, follow the call. With R-C confirmed, jumper or command the fan and cool calls and see what energizes. From here you've crossed from "totally dead" into a normal control-circuit trace — find where the 24V stops getting where it needs to go.
What "normal" looks like
- Line voltage: ~240V (208–250 acceptable) at the condenser disconnect and the air-handler whip; ~120V at a furnace's hot leg.
- Control voltage: 24–28 VAC across R-C at the board. Below ~21V and contactors/relays get flaky.
- Low-voltage fuse: intact, typically 3A or 5A automotive blade.
- Float switch: closed (continuity) when the pan is dry and the line is clear.
Common faults & what they mean
- Tripped condensate float / clogged drain: kills the entire 24V call, system goes dead silent. Clear the drain, confirm the switch resets, address why it clogged.
- Blown low-voltage fuse: a thermostat wire rubbed through, a shorted contactor coil, or a pinched wire at a condenser took out the transformer's protection. Find the short.
- Dead transformer: no 24V with good line voltage and an intact fuse. Often killed by the same short that should have blown a fuse on a unit that doesn't have one.
- Tripped or failed breaker / blown disconnect fuses: no line voltage to one or both units. A breaker that re-trips is a fault, not a nuisance.
- Open blower-door / panel safety switch: panel not seated after the last service. Quick to overlook, quick to fix.
- Failed thermostat or dead batteries: the stat never sends the call. Confirm it's actually lit and commanding.
Tech tips & gotchas
- Check the float switch early. It's free to check and it's the answer more often than any single component. A musty smell or water in the secondary pan is your hint before you even meter anything.
- A blown fuse is a symptom, not the disease. If you replace it and it pops again on the cool call, the short is on the Y circuit out to the condenser — likely a rubbed-through thermostat cable at the outdoor unit.
- Verify line voltage before you blame low voltage. No point chasing a transformer when the breaker's tripped.
- Don't forget the disconnect pull. Service techs (or critters) sometimes leave the outdoor disconnect pulled or partially seated.
- Wiggle-test the thermostat wires at both ends. A nicked R or C at a screw terminal makes an intermittent dead system that drives everyone crazy.
Safety / code notes
- Verify power is truly off with a meter before touching line-voltage terminals — don't trust the breaker label.
- The outdoor disconnect must be present and within sight of the condenser per NEC §440. If it's missing, that's a correction, not optional.
- When you find a clogged condensate drain, make sure the drain and pan meet local mechanical/plumbing code so the overflow protection works as intended.