AC Not Turning On in Mountain View

AC Not Turning On in Mountain View

When an AC won't turn on at all, people assume the worst. In most cases it isn't the compressor or the whole system. It's a single failed part in the start-up chain: a tripped breaker, a dead thermostat, a blown low-voltage fuse on the control board, or a capacitor that no longer has the punch to spin the fan and compressor up. The fix is usually inexpensive and same-visit.

Mountain View adds a wrinkle. A lot of the older homes here were built with heat only and had AC added later, a condenser and coil tied into the existing furnace. On those retrofits the cooling side runs off the furnace control board and a 24-volt circuit that was never originally sized for it. A blown board fuse, a loose low-voltage wire, or a clogged condensate float switch will shut the whole call for cooling down even though the furnace still heats fine.

Summers here stay on the mild side, so these systems often sit idle for months, then get asked to run on the first warm week. Contactors corrode and capacitors weaken while nothing is calling for them, so the failure shows up the day you finally need cooling. We see it every year, and it's almost always one part.

Common causes

Tripped breaker or pulled disconnect. The first thing we check. The outdoor unit has its own breaker and a disconnect block at the condenser. A breaker that trips immediately on reset points to a shorted compressor or fan motor; one that holds is often just a nuisance trip. We test current draw before assuming the worst.

Dead thermostat or batteries. A blank or frozen thermostat won't send a call for cooling. On older retrofits we also see miswired thermostats after a smart-stat swap, where the cooling wire was never landed. We meter the thermostat terminals to confirm the call is actually leaving the wall.

Failed capacitor. The most common real failure. A weak run capacitor leaves the fan and compressor unable to start, so you get a hum or nothing. We read the capacitor's microfarad value against the nameplate and replace it from truck stock. Typical $150 to $250 on the written estimate.

Blown low-voltage fuse on the furnace board. Common on Mountain View AC-added-to-furnace setups. A small board fuse protects the 24-volt circuit; a pinched wire or a shorted contactor coil blows it and kills the cooling call. We find the short rather than swap the fuse, so it doesn't blow again.

Tripped condensate float switch. If the drain backs up, a float switch cuts the system to prevent overflow. On retrofit installs the drain often runs long and shallow. We clear the line, flush it, and confirm the switch resets.

Worn contactor. The contactor is the relay that sends power to the condenser. Pitted or stuck contacts mean the outdoor unit never energizes. We inspect and replace it; it's a low-cost part that prevents a repeat no-start.

How we diagnose it

• Confirm the breaker and outdoor disconnect are set and holding, and read startup amp draw on the condenser.
• Meter the thermostat and the 24-volt circuit at the furnace board to see if the cooling call is actually reaching the equipment.
• Test the capacitor's microfarad value and the contactor for pitting or a chattering coil.
• Check the furnace board's low-voltage fuse and trace any short before replacing it.
• Inspect the condensate line and float switch, since a tripped float looks exactly like a dead system.

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

AC Not Turning On in Mountain View: common questions

Nearby and related

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

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