Safety first. If you smell gas (a rotten-egg smell), leave the house now. Don't touch light switches, the thermostat or appliances on the way out. Once you're outside, call 911 and SDG&E (natural gas) or your propane supplier. If a carbon monoxide alarm sounds, or anyone has headaches, dizziness or nausea that ease when they go outside, get everyone out into fresh air and call 911. Don't go back in until responders say it's safe.
When a furnace isn't working, start with simple checks: make sure the thermostat is set to HEAT and above room temperature, check the furnace breaker and power switch, replace a dirty filter, and make sure the furnace door is closed. If the furnace still won't heat, turn it off and call a technician. Never work on gas lines, burners or venting yourself.
Not sure what's wrong?
Try our quick troubleshooter →Fallbrook Heating & Air has repaired furnaces across North County since 1972. These are the same checks we'd walk you through on the phone. They're safe for any homeowner, and they often save you a service call.
What should I check first when the heat won't come on?
Check the thermostat, the power and the filter, in that order. Most "dead furnace" calls turn out to be a thermostat setting, dead thermostat batteries, a tripped breaker, a furnace switch someone turned off, or a filter so clogged the furnace shuts itself down to avoid overheating.
- Thermostat. Set it to HEAT, and set the temperature a few degrees above the room. If the screen is blank, replace the batteries. If it has a schedule, check it isn't holding a low setting.
- Fan setting. Set the fan to AUTO, not ON. On ON, the blower runs constantly and blows cool air between heating cycles.
- Breaker. Find the furnace breaker in your electrical panel. If it's tripped, switch it fully off, then on, once. If it trips again, leave it off and call us.
- Furnace power switch. Furnaces have a switch that looks like a light switch, on or near the unit. Make sure it's on.
- Furnace door. The front panel presses a safety switch. If it isn't seated properly, the furnace won't run.
- Filter. Pull the filter and hold it up to a light. If you can't see light through it, replace it.
- Condensate drain (high-efficiency furnaces). A 96% furnace drains water through PVC piping. If its drain or condensate pump is blocked or full, a safety switch can stop the furnace. Look for water around the unit.
After any fix, give the furnace a few minutes. Many furnaces run a short start-up sequence before the burners light and the blower starts.
Why is my furnace blowing cold air?
The most common reason is the thermostat fan set to ON, which runs the blower between heating cycles. If the fan is on AUTO and the air is still cold, the burners may not be lighting because of an ignition or flame-sensor problem, a gas supply issue, or an overheating shutdown from a dirty filter.
| Symptom | Likely causes | Safe homeowner step | Call a technician if… |
|---|---|---|---|
| Nothing happens at all | Thermostat setting or batteries, breaker, furnace switch, door switch | Check thermostat, breaker, switch and door | It still won't start, or the breaker trips again |
| Blower runs, air is cold | Fan set to ON; ignition or flame-sensor problem; gas supply | Set fan to AUTO; replace filter | Air stays cold on AUTO |
| Starts, then shuts off quickly | Dirty filter (overheating), dirty flame sensor, blocked vent | Replace filter; check outdoor vent for obvious blockage from a safe distance | It keeps short-cycling |
| Water around a high-efficiency furnace | Blocked condensate drain or failed pump | Turn furnace off; wipe up water | Water returns or the furnace won't restart |
| Burning smell on first use of the season | Dust burning off the heat exchanger | Usually fades within a short time | It persists, smells electrical, or smoke appears |
| Gas smell, CO alarm, or symptoms | Possible gas leak or carbon monoxide | Leave the house; call 911 and the gas utility from outside | Always. Don't restart the furnace |
What about the pilot light?
Most furnaces installed in recent decades don't have a standing pilot light; they use electronic ignition. If yours does have a pilot, relight it only by following the instructions on the furnace label, and only if there's no gas smell. If the pilot won't stay lit, a part such as the thermocouple likely needs a technician.
What should I never do myself?
Don't work on gas lines, gas valves, burners, venting or wiring inside the furnace, and don't bypass safety switches. These are safety systems, and mistakes can cause gas leaks, carbon monoxide or fire. They need a licensed technician. Replacing a filter and resetting a breaker once are fine; repeated trips are a warning sign.
- Don't tape down or jumper the door switch or any safety switch.
- Don't keep resetting a breaker that trips again.
- Don't use the oven or a gas stovetop to heat your home.
- Don't run fuel-burning space heaters or generators indoors or in an attached garage.
Do I need a carbon monoxide alarm?
If you have a gas or propane furnace, gas appliances or an attached garage, yes: keep working CO alarms outside sleeping areas and on each level, and test them regularly. A furnace with a cracked heat exchanger or blocked vent can release carbon monoxide, which you can't see or smell.
A yearly heating check includes looking at the burners, venting and heat exchanger for exactly these problems. In our mild climate, furnaces can sit unused for months, so a fall check before the first cold snap is a good habit. See our tune-ups.
What if I have a propane furnace?
Many rural homes in Fallbrook, De Luz and Rainbow heat with propane. The same checks apply, plus one more: make sure the tank isn't empty and the tank valve is open. If the tank ran out, call your propane supplier: they need to refill the tank and do any required safety checks before anything is relit.
We work on natural gas and propane furnaces, including wall and floor furnaces. Our Rainbow, De Luz and Pala Mesa page covers rural homes.
Is it time to repair or replace my furnace?
Most furnace problems are worth repairing: igniters, flame sensors, pressure switches and blower motors are common fixes. Replacement makes sense when the heat exchanger is cracked, repairs are frequent, or you're replacing your AC at the same time and want to consider a heat pump.
Start with our heating repair page. If you're weighing a new system, see furnace installation (we install 80% and 96% efficient furnaces) and our heat pump vs furnace guide.
“The guys who came out were great. Found the problem right away and repaired it.”
Frequently asked questions
Why is my furnace blowing cold air?
Often the thermostat fan is set to ON instead of AUTO, so the blower runs even when the burners aren't heating. Other causes include a dirty filter that makes the furnace overheat and shut its burners off, an ignition or flame-sensor problem, or a gas supply issue. If simple checks don't fix it, call a technician.
What should I do if I smell gas near my furnace?
Leave the house right away. Don't flip light switches, use the thermostat, start a car in an attached garage, or use your phone until you're outside. From outside, call 911 and your gas utility (SDG&E for natural gas, or your propane supplier). Don't go back in until they say it's safe.
Is it safe to relight my furnace pilot light?
Only if your furnace has a standing pilot and the instructions on the furnace label explain how, and only if there's no gas smell. Most modern furnaces use electronic ignition and have no pilot to relight. If the pilot won't stay lit, or you're unsure, call a technician rather than repeatedly trying.
Why does my furnace turn on and then shut off right away?
Short cycling is often caused by a dirty filter that makes the furnace overheat, a dirty flame sensor that can't confirm the flame, or a blocked vent. Replace the filter first. If it keeps happening, turn the furnace off and call a technician; repeated safety shutoffs are the furnace protecting itself.