An air conditioner that runs without cooling, short-cycles or will not start usually has one of a handful of problems: restricted airflow, a failed electrical part, a dirty outdoor coil, a blocked condensate drain or a refrigerant leak. Telling them apart takes measurements, not guesses — and it keeps you from paying for a refrigerant top-off when the real fault is a weak capacitor.
What we check first
Airflow before refrigerant. A clogged filter, closed or blocked returns, a dirty evaporator coil or a slow blower make the system look undercharged. We check the filter, the return, the blower and the temperature drop across the indoor coil before anyone connects gauges.
The electrical path. Capacitors, contactors, fuses, the disconnect and the thermostat wiring fail more often than compressors. We test them under load, along with the amp draw of the fan and compressor motors.
Heat moving outdoors. The outdoor coil has to shed the heat taken out of the house. Dryer lint, cottonwood fluff, leaves and dust pack the fins, and on a warm afternoon the system cannot keep up. We look at the coil and the outdoor fan before judging the refrigerant side.
Drains and safety switches. A clogged condensate line or a full secondary pan trips a float switch and shuts the system off, sometimes with water on the ceiling of the room below an attic unit.
Common symptoms and what they usually mean
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Outdoor unit hums but the fan does not spin | Failed run capacitor or fan motor | Capacitor or motor replacement |
| Runs constantly, house never reaches the setting | Dirty coil, low airflow or refrigerant leak | Cleaning, airflow correction or leak repair |
| Ice on the copper line or indoor coil | Low airflow or low refrigerant | Filter/blower fix or leak search |
| Water around the indoor unit | Clogged condensate drain | Drain clearing and float switch check |
| Clicks on and off every few minutes | Thermostat, oversized system or electrical fault | Testing before any part is changed |
| Breaker trips when the AC starts | Shorted motor, compressor or wiring | Electrical diagnosis |
If your system is making new noises, our page on AC noises explains which ones mean “turn it off now”.
What affects the price
Most AC repairs fall between $89 and $450. Electrical parts such as capacitors, contactors and fuses are at the low end and are usually fixed on the first visit. Fan motors, refrigerant leak searches and coil work cost more, and the type of refrigerant in the system matters. The diagnostic fee is waived when you go ahead with the repair, and you see the price before any work starts. For a fuller breakdown, see the AC repair cost guide.
Repair or replace?
Repair makes sense when the system has one clear failed part and otherwise cools well. Replacement is worth discussing when an older system needs a compressor or a coil, when it uses a refrigerant that is getting expensive to service, or when it has needed repeated repairs. Our repair-or-replace guide walks through the numbers, and if replacement is the better choice we cover AC installation and heat pumps too.
Before you call: safe things to check
- The thermostat is set to Cool and a few degrees below room temperature, with fresh batteries if it uses them.
- The filter is not grey and clogged.
- The breakers for the indoor and outdoor units are on, and the outdoor disconnect has not been pulled.
- The outdoor unit is not buried in plants or debris.
If the outdoor unit hums without the fan turning, or the breaker trips again, switch the system off and call — repeated restarts can damage the compressor.
Local notes
Summer mornings near the bay are often mild, but the warmest days in San Leandro, San Lorenzo and Castro Valley tend to come in late summer and early fall, when systems that sat idle for months suddenly run all afternoon. That is when weak capacitors and dirty coils show up. Many homes here have ductwork in the attic, so a room that never cools may need duct repair rather than a bigger unit. Hillside homes with large west-facing windows can also outgrow a system that was sized for a different layout.
A yearly AC tune-up in spring catches most of these problems before the first warm spell.