Furnace Won't Turn On? A Colorado Troubleshooting Guide

A furnace that "won't turn on" is usually missing one thing: power, a call from the thermostat, airflow, a clear vent, or gas. Here is each one in the order a technician checks them, what you can safely look at yourself, and when to stop.

Which kind of "won't turn on" do you have?

Stand by the furnace while someone raises the thermostat, and listen for two minutes.

• Nothing at all. No click, no hum, no fan. Think power: thermostat, service switch, breaker, door switch, or a blown low-voltage fuse.
• A small fan (the inducer) hums, then everything stops. The furnace is trying, but a pressure switch isn't proving a clear vent, or ignition is failing. Check the outdoor pipes and read the light code.
• You hear clicking and see a glow, but no flame. That points to the gas supply, the gas valve, or the igniter.
• The burners light, then shut off within seconds. That is classic flame sensor behavior. See why a furnace short cycles.
• The blower runs but the air is cool. That is a different problem with its own guide: furnace blowing cold air.

Symptom, likely cause and who should fix it

Step-by-step checks you can do safely

1. The thermostat

Confirm the mode says Heat and the setpoint is 3 to 5 degrees above room temperature. Replace the batteries if it has them, even if the screen looks fine, and check a smart thermostat's app for a forgotten eco, away or vacation hold. If the screen is blank, our thermostat troubleshooting guide covers Honeywell, ecobee and Nest.

2. The service switch and the breaker

Nearly every furnace has its own on/off toggle, mounted on the cabinet or a wall or joist close by, and people bump it off by accident all the time. Carrier's homeowner troubleshooting page also suggests checking the panel for a breaker labeled furnace or heat; if it sits in the middle position, push it fully off and then on. If that breaker trips a second time, leave it off and call.

3. The blower door

A small push-button switch behind the lower door cuts all power when that panel is off or not seated. After a filter change, it is easy to leave the door a quarter inch proud. Press it firmly into place. Do not tape or hold that switch in.

4. The air filter

Slide the filter out and look at a lamp through it. If almost no light passes, swap in a new one of the same size. A starved furnace overheats, and the high-limit switch shuts the burners off to protect the heat exchanger. Some furnaces then lock out until power is cycled. Carrier recommends turning the furnace power off before swapping the filter.

5. The gas supply

If the water heater, range and fireplace also don't work, the problem is the gas service, so call Xcel Energy. The furnace's own gas shutoff is a small handle on the pipe feeding it; on most quarter-turn valves the handle is in line with the pipe when open. If you are not sure, or the handle is in an odd position, leave it alone and ask a technician. Never try to light burners by hand on a modern furnace.

6. The outdoor vent pipes

Condensing furnaces, the 90%-plus kind, pull combustion air in through one plastic pipe and push exhaust out another. If either is buried, frosted shut, or has a bird nest or leaves in it, the pressure switch won't close and the furnace won't light. Clear snow and ice by hand from around the openings. Don't chip at the pipe with a shovel, and never put a cap or bag over the terminations.

7. The condensate drain

Those same furnaces make water. Look for water on the floor, a condensate pump that is full or not running, or a drain line through a cold garage or attic that may be frozen. A backed-up drain can hold the pressure switch open or trip an overflow switch. You can empty a full pump reservoir; thawing or rerouting a frozen line is a job for a technician.

8. The blinking light

Look through the small window in the lower door for a red, green or amber LED. Count the flashes or film it with your phone, and match the pattern to the chart on the inside of the door. Our guide to furnace blinking light codes explains how to read the different patterns. Do this before you cut the power.

9. One reset, not five

Once the obvious causes are fixed, turn the service switch off for about 30 seconds and back on. That clears a lockout and gives the furnace a fresh try. If it fails again with the same code, stop. Repeated resets on an ignition or rollout fault can let unburned gas build up or hide a heat exchanger problem.

Colorado-specific causes

Snow-drifted and frosted PVC vents

Front Range storms often drop heavy, wet snow, then blow it against the side of the house. Sidewall vents low on a north or west wall are the usual victims. A U.S. Department of Energy measure guideline on high-efficiency gas furnaces says vent terminals should sit one foot above the maximum expected snow level, which in most cases means about three feet off the ground. If yours are lower and keep getting buried, ask about raising them.

Frozen condensate lines

The same DOE guideline says condensate drains running through spaces that can fall below freezing need protection. Attic furnaces and drain lines that pass through garages or crawlspaces are the common ones to freeze during a subzero stretch.

Altitude

Thin air means less oxygen and weaker draft, so a furnace never set up for the elevation, or one with a sagging vent pipe, is more prone to pressure switch and ignition faults. Our HVAC altitude guide explains derating.

Wildfire smoke and dust

A smoky summer loads filters fast, so a filter changed in spring can trip the limit on day one of heating season.

Hail and storms

Hail can crack plastic vent caps, and storm outages and surges occasionally take out a control board or low-voltage fuse.

The first cold snap

A furnace that sat idle since April may have a dirty flame sensor, a mouse nest in a pipe, or a dead thermostat battery. A fall tune-up catches most of these before the night you need heat.

When to call a technician

• The breaker or the control fuse trips again.
• The same flash code returns after one reset.
• The inducer runs but the furnace never lights, and the vents are clear.
• A manual-reset rollout switch has tripped.
• Water is pooling and you can't find a simple cause.

MrFixer Heating & Cooling handles furnace repair across the Denver metro, and our emergency HVAC repair line is (720) 466-7983. See the areas we serve. If you're weighing the bill, our furnace repair cost guide covers repair versus replacement.

While you wait: an electric heater that shuts itself off if tipped or overheated is fine if kept three feet from anything that burns,. Never heat the house with a gas range or oven.

Frequently asked questions

The thermostat says Heat. Why won't my furnace start?

The usual reasons are a furnace service switch that's off, a tripped breaker, a blower door that isn't latched, a clogged filter that tripped the limit switch, or a blocked PVC vent. Check those, then read the blinking light code.

Where is the reset button on a furnace?

Many modern furnaces don't have one; you reset them by turning the service switch off for about 30 seconds. Some have a manual-reset rollout switch near the burners. If that one has tripped, call a technician instead of pressing it again.

Can a dirty filter stop a furnace from turning on?

Yes. Restricted airflow overheats the furnace, the high-limit switch shuts the burners off, and some models lock out until power is cycled. Replace the filter and give it one reset.

My furnace hums but won't light. What does that mean?

The inducer fan is running, but the control isn't getting proof of a clear vent or ignition is failing. On a high-efficiency furnace, check the outdoor intake and exhaust pipes for snow, ice or debris and look for a full condensate pump.

Is it safe to keep resetting my furnace?

No. One reset after fixing an obvious cause is fine. If the same fault returns, repeated resets can mask an ignition, venting or heat exchanger problem. Leave it off and call.

What should I do if I smell gas near a furnace that won't start?

Don't touch switches or the thermostat. Leave the house, move away, and call 911 and Xcel Energy at 1-800-895-2999 from outside.