TL;DR. Most furnace problems fall into 8 patterns. Flame sensor cleaning ($0-$300) solves about 40% of “no heat, blower runs” calls. Filter replacement solves most short-cycling and weak-airflow issues. Call us if: you smell gas, the blower won’t stop after thermostat is set to Auto, or a 12+ year-old furnace has component failures.
Direct answer, the top 8 furnace problems we diagnose
| Problem | Most likely cause | DIY-fixable? | Pro cost if needed |
|---|---|---|---|
| No heat, blower runs | Failed flame sensor or igniter | Maybe (sensor cleaning) | $200-$400 |
| No heat, blower silent | Thermostat, control board, or no power | Maybe (thermostat batteries) | $300-$1,200 |
| Short cycling (on/off rapidly) | Dirty filter, oversized furnace, bad limit switch | Filter yes, rest no | $200-$1,200 |
| Blower runs constantly | Thermostat set to “on” instead of “auto” | Yes (check thermostat) | $0 |
| Weak airflow | Dirty filter, blocked vents, or duct issues | Filter and vents yes | $300-$1,800 |
| Weird burning smell | Dust burning off after summer, or worse | Wait one cycle, then call pro if smell continues | $200-$2,500 |
| Banging or rumbling | Delayed ignition, blocked burner, ductwork | No | $300-$900 |
| High utility bill | Aged filter, system inefficiency, ductwork loss | Filter yes | $0-$1,500 |
No heat but the blower runs
The blower running without heat means the furnace is “trying” but failing to ignite or sustain the flame.
Most likely causes:
- Failed flame sensor (most common, 60%+ of cases). The flame sensor is a metal rod that confirms the burner ignited. When it fails, the gas valve closes for safety. Cleaning the sensor with steel wool often restores function for another 6 to 12 months.
- Failed igniter (hot surface igniter or pilot). Newer furnaces use hot surface igniters that crack and fail every 5 to 8 years.
- Dirty or failed pilot tube (older furnaces).
DIY check:
- Pull the front panel of the furnace and look at the burner area. If the flame sensor (small metal rod in front of the burner) is coated with white residue, gently clean with fine steel wool or sandpaper. Reinstall and test.
- This is a $0 fix that resolves about 40% of “no heat, blower runs” calls.
When to call:
- Cleaning the sensor doesn’t fix it
- You smell gas (always call immediately)
- Older furnace with pilot light not staying lit
No heat, blower silent
The blower not running means the furnace isn’t getting the signal to start, or isn’t getting power.
Most likely causes:
- Dead thermostat batteries
- Tripped breaker at the panel
- Failed control board on the furnace
- Failed transformer
- Furnace switch turned off (yes, this happens)
DIY check:
- Verify thermostat displays a temperature and is set to heat mode
- Replace thermostat batteries if applicable
- Check the breaker labeled “furnace” at the panel; reset if tripped
- Check the wall switch near the furnace; sometimes turned off during summer maintenance
When to call:
- All of the above check out and the furnace still won’t start
- Breaker trips again after reset (electrical issue requiring diagnosis)
Short cycling
The furnace turns on, runs for 3 to 8 minutes, shuts off, then starts again 2 to 5 minutes later.
Most likely causes:
- Dirty air filter restricting airflow
- Oversized furnace heating the home too quickly
- Failed limit switch shutting the system down for safety
- Failed flame sensor (intermittent, comes back on after cooling)
DIY check:
- Replace the air filter. If it’s the dark gray “I haven’t changed this in 6 months” filter, that’s likely the cause.
- Wait one heating cycle and observe.
When to call:
- Filter replacement doesn’t resolve
- Furnace is over 12 years old (signals component failure)
- You notice the home heating unevenly (cold rooms while others are warm)
Blower runs constantly
The blower runs nonstop, even after the burner shuts off. The home isn’t getting hotter; the blower just won’t quit.
Most likely cause:
- Thermostat set to “on” instead of “auto.” The blower runs continuously, blowing room-temperature air. This is the most common cause and the easiest fix.
Other causes:
- Failed fan limit switch (rare, $250 to $400 repair)
- Wiring short on the blower circuit (rare)
DIY check:
- Open your thermostat and look for “Fan: On / Auto” setting. Switch to “Auto.”
When to call:
- Thermostat is set to Auto and blower still runs continuously
- Blower runs even when system is set to “Off”
Weak airflow from vents
Air comes out of the vents but feels weak or not very warm.
Most likely causes:
- Dirty air filter restricting flow
- Closed or blocked vents (furniture, rugs over floor registers)
- Ductwork leaks (common in Seattle homes with old, brittle duct mastic)
- Ductwork sizing issues (undersized for current furnace)
- Failed blower motor capacitor (motor running at reduced speed)
DIY check:
- Replace filter
- Verify all supply registers are open
- Walk through the home and feel each register; if some are strong and some are weak, you have a duct issue. If all are weak, you have a system-wide issue.
When to call:
- Filter replacement doesn’t fix
- You see disconnected ducts in the basement or attic
- Blower motor sounds different than normal (humming, grinding)
Weird burning smell from vents
Smell coming from the vents when the furnace first runs each fall.
Most likely cause:
- Dust burning off the heat exchanger after a summer of disuse. Completely normal. Open windows for 10 to 15 minutes during the first burn. Smell should be gone after one heating cycle.
More serious causes (if smell persists):
- Electrical burning (wiring insulation failing): turn off the furnace at the switch, call us
- Plastic or chemical burning: turn off at the switch, call us
- Mold smell: usually a duct or condensate issue, call us
Carbon monoxide concern:
- CO has no smell. It’s not a smell issue. If your CO detector goes off, evacuate and call 911.
Banging or rumbling
Loud bang on startup, or continuous rumbling during operation.
Most likely causes:
- Delayed ignition: gas builds up briefly before ignition, then ignites with a bang. Caused by dirty burner or weak igniter. $300 to $500 repair.
- Sediment in the heat exchanger
- Loose ductwork expanding and contracting (common in older homes)
Always call:
- A loud bang on startup is a safety issue. We diagnose same-day.
High utility bill
Heating costs are noticeably higher than previous winters.
Most likely causes:
- Dirty filter forcing the system to work harder
- Furnace aging (10+ year old systems lose 1 to 2% efficiency per year)
- Thermostat malfunction (calling for more heat than registered)
- Ductwork losses (especially in unconditioned attic or crawl space)
- Insulation degradation in attic or walls
DIY check:
- Replace filter
- Verify thermostat is reading correctly (use a separate thermometer in the same room)
- Check attic or crawl space ducts for visible disconnects
When to call:
- DIY checks don’t explain the increase
- System is over 15 years old (consider replacement; see when to replace furnace Seattle)
Schedule a furnace diagnostic
Call (206) 200-9134. Same-day diagnostic Mon-Sun, 7 AM to 8 PM. $129 diagnostic fee credited toward repair.