Why is my furnace not heating?

A furnace that won't heat is most often a dirty filter choking airflow, a failed igniter, or a thermostat problem — in that order of cheapness. A furnace that starts, runs a minute, then stops is usually a dirty flame sensor. True no-heat in deep cold justifies the after-hours call; a mild-weather failure usually doesn't.

Likely causes

• Clogged filter — Restricted airflow overheats the furnace until the safety switch shuts it down.
• Failed igniter — Modern furnaces light with a hot-surface igniter that cracks every 5–7 years. Clicking with no flame points here.
• Dirty flame sensor — The furnace lights, runs a minute, and quits — the sensor can't prove flame, so it shuts down safely.
• Thermostat or power issue — Dead thermostat batteries, a tripped breaker, or an open furnace door switch — check before you pay for a visit.

What you can check yourself

• Replace the filter, and confirm the thermostat is on 'heat' with fresh batteries if it uses them.
• Check the breaker and the furnace's own power switch (it looks like a light switch, usually near the unit).
• Make sure the furnace access door is fully closed — many units won't run with the door switch open.

Typical cost

Typical repairs in Los Angeles, CA: service call $100–$250; igniter $150–$350; flame sensor cleaning/replacement $150–$300; blower motor $400–$800.

When to call a pro

Gas smell, a carbon monoxide alarm, or a furnace that short-cycles with a burning smell — leave and call from outside. Otherwise, no heat below ~40°F with kids or elderly at home counts as urgent.

HVAC pros in Los Angeles, CA

• Accelerate Heating & Ac Repair · 333 S Figueroa St, Los Angeles, CA · 14242735345
• Complete Energy And Air · 453 S Spring St Ste 400, Los Angeles, CA · +13106955814
• Los Angeles Heating and Air Conditioning · 114 San Pedro St, Los Angeles, CA · +12136953895
• Alaric HVAC AC Service Loss Angeless · 707 N Broadway, Los Angeles, CA · +12134744475
• Los Angeles HVAC Repair Service · 1005 W Rondout St, Los Angeles, CA · +12133761525
• Southwest Heating & Air Conditioning · 622 Alpine St, Los Angeles, CA · +13108670703
• Delux AC Repair Los Angeles · 820 S Grand Ave, Los Angeles, CA · +14243095123
• Bgreenla · 1055 Wilshire Blvd Ste 1475, Los Angeles, CA · (213) 353-3982

Get free quotes from Los Angeles, CA pros →

Data by Servik — verified local pros from official license records and public sources. Cost figures are typical ranges, not quotes; a pro's written estimate is the real number.