HVAC Guide

Furnace Not Working? The No-Heat Troubleshooting Checklist for California Homes

Updated 2026-02-19 • By the Air Conditioning & Heating Repair California technical team

California furnaces sit idle for eight or nine months, then get asked to run flawlessly on the first cold night. That's why the biggest no-heat spike of the year happens in the first two weeks of real cold. Here's how to triage it.

Step 1: Confirm it's calling for heat

Thermostat set to HEAT, setpoint above room temp, fresh batteries installed.
Check the furnace switch on the wall or side of the unit — it looks like a light switch and gets flipped off by accident constantly.
Check the breaker, and check that the furnace door panel is fully seated (a loose door opens a safety switch).

Step 2: Airflow and filter

A filter left in since last spring is the #1 cause of furnace short cycling and limit-switch lockouts. Restricted airflow overheats the heat exchanger, the high-limit trips, and the furnace shuts down after a few minutes of heat. Replace the filter and see if the cycle normalizes.

Step 3: Read the blink code

Most modern furnaces flash a diagnostic code through a small window. Count the blinks and check the legend inside the door. Common ones we see:

Ignition lockout - failed hot surface ignitor or dirty flame sensor.
Pressure switch open - blocked flue, condensate clog, or failing inducer motor.
Limit switch open - airflow restriction, dirty blower, or closed vents.
Flame sensed with no call for heat - gas valve or board fault. Shut it down and call.

Step 4: Know the stop signs

Shut the system off and call immediately if you smell gas, see soot or rust streaks around the burner compartment, hear a boom on ignition, or your CO alarm sounds. A cracked heat exchanger is a life-safety issue, not a comfort issue. Combustion safety testing is included on every furnace repair we perform.

What a proper heating repair visit includes

Full sequence-of-operation test, not just a part swap
Gas pressure and temperature-rise measurement
Flame sensor cleaning and ignitor resistance check
Static pressure and blower amp draw
Carbon monoxide test at the supply plenum
Written findings and upfront repair pricing

No heat tonight? We answer 24 hours a day for emergency heating repair anywhere in California. Call (209) 305-4521, or find your city on our locations page.

Need help now? We answer 24/7 for same day AC repair, furnace repair and emergency HVAC service in every California city. Call (209) 305-4521 or find your city.