Furnace Not Working / No Heat? A Step-by-Step Troubleshooting Guide
By the technicians at Efficient Air Conditioning & Heating Repair · Updated 2026-08-23
A furnace that won't heat is more than an inconvenience — in freezing weather it becomes a burst-pipe risk. Here is the same diagnostic order our technicians follow.
Step 1 — Thermostat
Set to HEAT, raise the setpoint 5 degrees above room temperature, and replace the batteries. A dead thermostat is the most common “broken furnace” of all. Confirm the schedule isn't holding a low setback.
Step 2 — Power and the door switch
Check the breaker and the furnace service switch on the wall or unit (it looks like a light switch). Also make sure the blower door is fully seated — a door safety switch will keep the furnace off.
Step 3 — Filter and airflow
A clogged filter causes the furnace to overheat and trip its high-limit switch. Classic symptom: it runs a few minutes, then shuts off and won't restart for a while — a form of short cycling.
Step 4 — Gas supply and pilot/igniter
Verify the gas valve is open and other gas appliances work. Modern furnaces use a hot surface igniter or spark ignition — no standing pilot. A cracked igniter is a very common, inexpensive, same-day repair.
Step 5 — Read the blink code
Through the sight glass you'll see an LED flashing a fault code. Write down the pattern and the model number — it lets us bring the right part on the first trip. Common codes point to:
| Symptom | Likely cause |
|---|---|
| Clicks, no flame | Failed hot surface igniter or flame sensor |
| Runs then shuts off | Dirty flame sensor, overheating, or blocked flue |
| Blower runs constantly, cool air | Control board, limit switch, or thermostat wiring |
| Loud boom on startup | Delayed ignition — shut it down and call immediately |
| Water around a high-efficiency unit | Clogged condensate trap or blocked drain |
Repairs we complete same day
- Hot surface igniters and flame sensors
- Inducer and blower motors
- Gas valves, pressure switches and rollout switches
- Control boards and transformers
- Thermostats and low-voltage wiring faults
- Condensate pumps and trap cleaning
See furnace repair, heating repair or heat pump repair, then pick your city from our service area list. 24-hour dispatch: (866) 652-0413.
Frequently Asked Questions
Why does my furnace blow cold air?
Usually a failed igniter or flame sensor (no flame), a tripped high-limit from restricted airflow, or a thermostat set to fan ON. Heat pumps also blow cool air during defrost cycles, which is normal.
What is the most common furnace repair?
Igniters, flame sensors and capacitors, followed by pressure switches and control boards. Most are same-day fixes when the part is on the van.
Is no heat considered an emergency?
Yes — in freezing weather no heat risks frozen pipes and health problems for children and seniors. We run 24 hour heating repair for exactly this reason.
How often should a furnace be serviced?
Annually, before heating season. A furnace tune-up catches cracked igniters, weak capacitors and blocked flues before they leave you without heat.