AC Not Turning On in Los Altos

AC Not Turning On in Los Altos

An AC that won't turn on at all is rarely a dead system. Most of the time the compressor and coil are fine and one electrical part has quit: a weak run capacitor, a pitted contactor, a thermostat with no power, or a tripped breaker. Any one of those keeps the whole unit silent. We start there and prove it with a meter before anyone talks about replacement.

Los Altos adds a wrinkle because a fair number of the larger homes here run two zones off two separate outdoor units. One zone can go silent while the other still cools, so a room reads hot and people assume the whole system died. More often one condenser's contactor or capacitor failed and the other is fine. We test the unit that's quiet and the one that's running, so you only pay to fix what actually broke.

Climate matters less here than in the hot Tri-Valley cities. Summers stay mild with marine moderation, so the AC doesn't run flat out for months. That cuts both ways. Equipment that sits idle through a cool spell can fail to start the first time it's asked to run hard, and a home that gained a second story years ago may be leaning on a unit that was never sized for the added load.

Common causes

Failed run capacitor. The most common no-start cause we find. The capacitor gives the compressor and fan motor the jolt they need to spin up. When it weakens, you get a hum, a click, or nothing. We read it with a meter against its rated microfarads and swap it on the spot. Common parts ride on the truck.

Pitted or stuck contactor. The contactor is the relay that lets high voltage into the condenser. Its contacts burn and pit over years of cycling, and eventually it won't close. On a two-zone home only one zone goes dead, which throws people off. We test for voltage across the contactor and replace it if it's failed.

Tripped breaker or pulled disconnect. AC condensers run on a dedicated breaker plus an outdoor disconnect. A breaker that trips on startup, or a disconnect left half-pulled after yard or service work, kills power entirely. We check both first because they cost nothing to fix. A breaker that keeps tripping, though, is a symptom of a deeper electrical fault, and we find that before resetting it again.

Dead thermostat or low-voltage fault. A thermostat with dead batteries, or a low-voltage fuse blown on the air handler board, leaves the system with no call for cooling. On the larger homes with multiple thermostats, one zone's stat can die while others work. We check stat power, the 24-volt fuse, and the wiring at both ends.

Locked or seized compressor. Less common, but real on older systems. The compressor draws too much current trying to start and trips out, sometimes with a hum then silence. We measure startup amperage and check windings. If the compressor is genuinely locked, we lay out repair-versus-replace numbers honestly before anyone spends money.

How we diagnose it

• Confirm power: breaker, outdoor disconnect, and 24-volt control fuse on the board before touching anything else.
• Read the capacitor against its rated microfarads and test the contactor for proper closing and voltage pass-through.
• Check the thermostat for power and a real cooling call, and on two-zone homes verify which zone is actually down.
• Measure compressor startup amperage and winding resistance if the electrical front end checks out clean.
• Put the diagnosis and the repair options on a written estimate before any work starts.

$75 diagnostic, credited toward any repair over $200. You get a written quote before any work begins.

AC Not Turning On in Los Altos: common questions

Nearby and related

AC Not Turning On near Los Altos: Palo Alto · Mountain View · Cupertino .

This is usually a ac repair in Los Altos job. See our ac repair overview or the Los Altos service area.