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 San Diego, 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 San Diego, CA
- Legacy Plumbing Heating & Air · 2534 State St Ste 457, San Diego, CA · 6194530365
- HVAC NOW California · 550 W Date St Ste 312, San Diego, CA · (619) 333-6356
- Ray Williams Refrigeration & Air Conditioning Inc. · 737 17th St, San Diego, CA · +16192333388
- SAN DIEGO MECHANICAL PRO · 415 LAUREL STREET 427, SAN DIEGO, CA · (858) 376 7506
- ROBERTSON MECHANICAL INC · P O BOX 7098, SAN DIEGO, CA · (619) 840 9191
- Carini Home Services · 825 Washington St, San Diego, CA · 6193325176
- San Diego Heating & Air Conditioning · 1286 University Ave 279, San Diego, CA · 6192724221
- Big City HVAC and Appliance Repair · 3113 Market St # A, San Diego, CA · +16195418118
Get free quotes from San Diego, 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.