What you'll see
The system starts, runs for a short stretch, then shuts off — and does it over and over. "Short" matters: a unit cutting off after 30–90 seconds is failing on a safety (pressure switch, overload). A unit running 3–5 minutes and quitting is usually a control or sizing issue. A unit that runs a while, trips, and won't restart for minutes is hitting a thermal protector. Time the cycles first; the duration tells you which world you're in.
The trap here is comfort complaints get blamed on "the AC is broken" when an oversized system that satisfies too fast is doing exactly what bad sizing forces it to do.
Walk it in order
- Time it. Pull out a watch. How long does it run, and how long is it off before it restarts? Write down both. Seconds-long runs point to safeties; minute-long runs point to controls or load.
- Very short runs (under ~2 min), compressor trips off → suspect a pressure switch or the compressor overload. Put gauges on and run it. If head climbs fast and it cuts out, the high-pressure switch is doing its job — find why head is high (dirty condenser, dead/slow fan, overcharge, recirculating air, non-condensables). If suction crashes and it cuts out, the low-pressure switch is tripping — undercharge, restriction, or low airflow dropping coil pressure.
- Watch the contactor and 24V. If the contactor drops out and back in rapidly, the call itself is being interrupted — chattering contactor, loose Y wire, a safety opening and closing, or a thermostat sending a stuttering signal. Meter 24V at the contactor coil through a cycle.
- Check the thermostat side. A thermostat in a bad spot (in a draft from a supply register, in direct sun, near the return) satisfies and re-calls quickly. A too-tight differential/CPH setting, or a wired-up "fan delay" gone wrong, can do it too. Confirm the stat isn't mounted where its own system's air hits it.
- Consider sizing and airflow. A grossly oversized system cools the thermostat's little spot fast, satisfies, shuts off, then the house heat soaks back and it re-calls — classic 5-to-8-minute cycling with poor humidity control and cold-then-clammy comfort. Low airflow can also cause cycling by freezing the coil or tripping limits.
- Rule out electrical heat/overload. A compressor cycling on its internal overload (runs, gets hot, trips, cools, restarts) often ties back to high head, low voltage, a weak capacitor, or an overamping motor. Clamp the amps through a cycle.
What "normal" looks like
- Healthy cooling run time: typically several minutes to continuous on a hot day; a properly sized system runs long and steady in design conditions, not in quick bursts.
- Differential: a typical thermostat swings around ~1°F before re-calling; cycles-per-hour settings cap how often it can restart.
- Anti-short-cycle delay: many systems enforce a ~3–5 minute minimum off time to protect the compressor — a unit that respects this and still re-calls fast is a load/control issue, not the delay.
- Pressures: if it's tripping a switch, you'll see head or suction run to the cutout value, not sit normal.
Common faults & what they mean
- High-pressure trip: seconds-to-minutes runs, head spikes. Dirty condenser, failed/slow condenser fan, overcharge, recirculating air, or non-condensables in the system.
- Low-pressure trip: suction crashes and cuts out. Undercharge/leak, restriction, low airflow over the evaporator, or running too cold outside.
- Compressor overload cycling: runs, gets hot, trips, restarts after cooldown. High head, low line voltage, weak run cap, or a struggling compressor.
- Chattering contactor / loose low-voltage: rapid contactor in-out, very fast cycling. Bad contactor, loose Y/C, or an intermittent safety.
- Thermostat placement / settings: comfortable-but-annoying cycling, no pressure or electrical fault. Relocate the stat or fix the differential.
- Gross oversizing: short, frequent runs with humidity complaints. Not a repair — a design problem to explain honestly.
Tech tips & gotchas
- Time the cycle before you theorize. Thirty-second cuts and five-minute cuts are completely different failures. Don't guess the duration.
- A pressure switch is doing its job, not malfunctioning. When it trips, the fault is why the pressure got there — chase the pressure, don't bypass the switch.
- Never jumper out a safety to "make it run." Bypassing a high- or low-pressure switch to keep a customer cool can grenade a compressor. If you must verify the switch itself is bad, do it deliberately and briefly with gauges on.
- Oversizing is a conversation, not a part. If the equipment is twice the load, no repair fixes the cycling — explain it as a comfort/sizing reality and offer real options.
- Watch the amps and the pressures together through one full cycle. The moment of cutout tells you which protector fired.
Safety / code notes
- De-energize before opening panels; bleed capacitors before handling.
- Don't defeat pressure or temperature safeties as a "fix" — they protect the compressor and, with A2L refrigerants, are part of the system's safety design.
- Recover refrigerant per EPA 608 (40 CFR Part 82, Subpart F) if you open the system to address a charge or restriction cause.