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 Phoenix, AZ: 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 Phoenix, AZ
- Advanced Air Llc · 2410 S Central Ave, Phoenix, AZ · 14802197866
- Southland Industries · 246 E Watkins St, Phoenix, AZ · 4804234444
- Mosaic · 2800 N Central Ave, Phoenix, AZ · +16235128868
- Arizona Heat Pump Council · 2702 N 3rd St, Phoenix, AZ · +16022487767
- Sunrise Mechanical Inc · 125 N 2nd St Ste 110#354, Phoenix, AZ · 6027141009
- Good Guys HVAC · 101 E Jefferson St, Phoenix, AZ · +16027638503
- American Cooling And Heating · 40 N Central Ave Ste 1400, Phoenix, AZ · 6023413112
- Arizona Air Coolers LLC · 111 E Maricopa Fwy, Phoenix, AZ · +16232021395
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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.