When an air conditioner in Sunnyvale, Santa Clara or Cupertino stops cooling, the cause is usually on a short list: an airflow restriction, a failed electrical part, a dirty coil, a blocked drain or a refrigerant leak. Telling them apart takes readings, not guesses. A refrigerant top-off will not fix a weak capacitor, and a new capacitor will not fix a leak.
The order we check things
Each step rules out a cheaper, more common cause before moving on to an expensive one.
- Thermostat demand. Is the thermostat actually calling for cooling, and does the signal reach the indoor and outdoor units? Dead batteries, a loose wire or the wrong mode end some calls right here.
- Airflow. Filter, return grilles, blower speed and the evaporator coil. Poor airflow makes a healthy system look low on refrigerant and can freeze the coil.
- Outdoor unit operation. Does the condenser fan spin? Does the compressor start, or hum and stop?
- Capacitor and contactor. We measure the capacitor against its rating and look for pitted or stuck contacts on the contactor. These parts fail far more often than compressors.
- Coil condition. A matted outdoor coil cannot shed heat on a hot afternoon; a dirty indoor coil chokes airflow.
- Refrigerant clues. Pressures and temperatures, plus oily spots at joints and valves, show whether the charge is low and where to look for the leak.
- Compressor. Only when everything else checks out do we judge the compressor itself, with amp draw and winding tests.
Symptoms and what usually causes them
| What you notice | Common cause | Typical fix |
|---|---|---|
| Indoor fan blows, outdoor unit is silent | Contactor, capacitor, tripped breaker or low-voltage fault | Electrical repair |
| Outdoor unit hums, fan does not turn | Weak run capacitor or seized fan motor | Capacitor or fan motor replacement |
| Air is cool but weak at the vents | Dirty filter, blower problem or leaking ducts | Airflow work or duct repair |
| Frost on the large copper line | Restricted airflow or low charge | Airflow fix or leak search |
| Water near the furnace or air handler | Clogged condensate drain | Drain clearing and float switch test |
| Short on-off cycles | Thermostat, oversized system or a tripping safety switch | Testing before any part is changed |
Ice that keeps coming back is covered in more depth in why an AC freezes up, and odd sounds in what AC noises mean.
Brand-aware repair
Lennox, Carrier, Daikin, Trane, Rheem, Goodman and Mitsubishi systems differ in part availability and in how their controls behave. Some newer variable-speed systems use communicating thermostats and control boards, so a generic part may not work with them. We look up the exact model before ordering and tell you when a manufacturer warranty claim is worth making. Brand notes: Lennox, Carrier, Daikin, Trane, Rheem.
Refrigerant: find the leak first
Air conditioners do not use up refrigerant. If the charge is low, it escaped somewhere — often at a coil, a valve core or a line that rubs against something. Adding refrigerant without finding the leak means paying for the same refrigerant again later. Older systems that still use R-22 are harder to service because that refrigerant is no longer produced, which weighs on the repair-or-replace decision. More on that in refrigerant recharge and our note on refrigerant changes.
What shapes the repair scope
You get the findings and the cost factors before any work begins. What moves the scope:
- Access — an air handler in a tight attic or a condenser on a townhome roof takes longer to reach.
- Diagnostic time — an intermittent fault may need the system run through several cycles.
- The failed part — capacitors and contactors are often fixed on the first visit; compressors and coils are not.
- System age and refrigerant type.
- Timing — an urgent call during a heat spell.
Repair before replace
A system that cools well apart from one failed part is worth repairing. Replacement becomes a fair question when an older system needs a compressor or a coil, still runs on R-22, or has needed one repair after another. In that case we lay out both paths side by side, and AC installation explains what a replacement involves. Repairs are backed by a warranty covering both parts and labor.
Quick checks before you book
- The thermostat is on Cool and set below room temperature.
- The filter is not grey and clogged.
- The breakers are on and the outdoor disconnect is in place.
If the outdoor unit hums without the fan turning, switch the system off; repeated restarts can overheat the compressor. When cooling is completely out on a hot day, see emergency AC repair, and for the whole range of cooling work, air conditioning services. City pages: Sunnyvale, Santa Clara, Cupertino.