Furnace Not Heating in Cupertino

Furnace Not Heating in Cupertino

Cupertino's winters are mild, so furnaces here don't get pushed the way they do inland. That changes the diagnosis. When a furnace stops heating in Cupertino it is rarely from being overworked; it is usually a part that simply aged out. The system still wants to run. It just can't complete ignition or hold the flame.

Most of the housing here is mid-century single-story ranches with gas furnace and AC split systems, and the equipment that replaced the originals is now reaching the end of its own life. On furnaces of that age the cause is almost always one defined part: a cracked hot surface igniter, a carboned flame sensor, a limit switch tripped by a dirty filter, or a control board that has lost a relay. None of those means the system is dead.

We keep the diagnosis concrete: which part failed, what it costs, and whether the furnace is worth keeping. On a sound unit we fix the part. On an end-of-life system, Cupertino's mild winters mean a heat pump holds full capacity easily, so we put that option on the estimate too.

Common causes

Cracked hot surface igniter. The standard no-heat failure on modern gas furnaces. The igniter cracks from heat cycling and stops glowing, so the burners never light. We confirm with a continuity test and replace it.

Dirty flame sensor. The furnace lights and shuts off within seconds because the carboned sensor can't prove the flame, so the board closes the gas for safety. Cleaning it usually restores operation. We replace only a degraded sensor.

Limit switch tripped by a dirty filter. A clogged filter restricts airflow, the heat exchanger overheats, and the limit switch cuts the burners. It looks like a dead furnace but isn't. We check the filter, clear the restriction, and confirm the switch resets and holds.

Control board failure. On older equipment the board can lose the ignition-sequence relay and leave the furnace dead. We read the fault code and confirm the board is the real failure rather than wiring or a sensor.

Thermostat or wiring fault. With the smart and high-end thermostats common in Cupertino homes, a miswired or failed thermostat can stop the heat call from ever reaching the furnace. We verify the call at the board, so we don't replace a healthy furnace part for a thermostat problem.

Blower motor failure. The burners light but no warm air reaches the rooms because the blower motor or its capacitor has failed. We test the windings and capacitor and confirm operation on the heating speed.

How we diagnose it

• Confirm the thermostat is genuinely calling for heat and the call reaches the control board.
• Watch the full ignition sequence and read the board fault code.
• Check the filter and airflow for a limit-switch trip.
• Run combustion analysis, CO testing, and a heat exchanger inspection on the gas furnace.
• Document the failed part and the price, and the repair-versus-conversion math on aged equipment, in a written estimate.

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

Furnace Not Heating in Cupertino: common questions

Nearby and related

Furnace Not Heating near Cupertino: Sunnyvale · Saratoga · Los Altos .

This is usually a furnace repair in Cupertino job. See our furnace repair overview or the Cupertino service area.