Furnace Won’t Turn On: A Denver Homeowner’s Checklist Before Calling for Repair

It is the first cold night of the season. You turn the thermostat up and nothing happens. No click, no rumble from the basement, no warm air from the vents.

Before you book a repair, take 10 minutes to work through this checklist. A good share of “no heat” calls end with a simple fix: a switch that got turned off, a thermostat on the wrong setting, a furnace door that was not closed tightly, or an exhaust pipe blocked by snow. You can check all of these safely yourself.

If none of the steps below bring your heat back, you will at least know what to tell the technician. That can make the visit faster and cheaper.

Safety first: when to stop and call right away

Do not troubleshoot if any of these are true:

• You smell gas (rotten eggs). Leave the house now. Do not flip light switches or use the garage door opener. From outside, call 911 and then Xcel Energy’s gas emergency line at 800-895-2999. See Xcel Energy’s gas emergency guidance.
• Your carbon monoxide alarm is going off. Get everyone out, including pets, and call 911. Read our guide on how to tell if your furnace is leaking carbon monoxide.
• You smell burning plastic or electrical burning, or see scorch marks near the furnace.
• The breaker trips again right after you reset it.
• There is water pooling around or under the furnace.

For these, turn the furnace off at its service switch (if it is safe to reach) and call us at 303-645-4889. We offer emergency furnace repair across the Denver metro.

The 10-step checklist

Work through these in order. Each step takes a minute or two.

Step 1: Check the thermostat settings

This sounds too simple, but it is the most common fix.

• Make sure the system mode is set to Heat, not Off, Cool, or Auto.
• Set the temperature at least 3 to 5 degrees above the current room temperature.
• If you have a smart thermostat, check for an active schedule, “Eco” or “Away” mode, or a temporary hold that is overriding your change.
• Wait 2 to 3 minutes. Many thermostats have a built-in delay to protect the equipment.

Quick test: Switch the fan setting from Auto to On. If the furnace blower starts and air moves from the vents, the thermostat and the furnace both have power. The problem is likely in the heating side of the furnace itself. Switch the fan back to Auto when you are done.

Step 2: Look for a blank screen or low battery warning

A blank or dim thermostat screen means it has lost power.

• Replace the batteries (many use AA or AAA).
• If your thermostat has no batteries, it gets power from the furnace through a common wire (the “C-wire”). A blank screen then often means the furnace itself has no power. Go to Step 3.
• If you recently installed a new smart thermostat and the heat stopped working soon after, the wiring is a likely cause. Our post on whether smart thermostats really save money in Denver winters covers setup mistakes to avoid.

Step 3: Check the furnace power switch

Most furnaces have a service switch that looks like a normal light switch. It is usually on the wall or ceiling joist near the furnace, or on the side of the furnace. In many Denver basements it sits right next to the light switch for the stairs, so people turn it off by mistake.

• Make sure it is in the On position.
• Some switches have a red cover plate. That is the one.

Step 4: Check the circuit breaker

Go to your electrical panel and find the breaker labeled “furnace” or “FAU.”

• A tripped breaker sits in the middle position or looks different from the others.
• Push it fully to Off, then back to On.
• If it trips again, stop. A breaker that keeps tripping points to an electrical fault in the blower motor, inducer motor, or wiring. That needs a technician.

Step 5: Make sure the furnace door is closed tightly

Furnaces have a safety switch behind the lower door (the blower door). If the door is loose or crooked, the furnace will not run. This is very common right after someone changes the filter.

• Remove the door and put it back on firmly. Make sure it seats flat on all sides.
• Listen for the furnace to start within a minute or two.

Step 6: Check the air filter

A badly clogged filter blocks airflow. The furnace then overheats, and a safety switch called the high-limit switch shuts the burners off. After this happens a few times, many furnaces lock themselves out completely.

• Pull the filter and hold it up to a light. If you cannot see light through it, replace it.
• In Denver, dust, dry air, pets, and wildfire smoke can load a filter faster than the package says. Our guide on which furnace filter to use and how often to change it explains what works best here.

After replacing a clogged filter, you may need to reset the furnace (Step 9).

Step 7: Confirm your gas supply is on

• Are other gas appliances working? Try the gas stove or check that the water heater has hot water. If none of them work, the problem may be with your gas service, not the furnace. Call Xcel Energy.
• Check the gas shutoff valve on the pipe going into the furnace. The handle should be parallel to the pipe (open). If it is across the pipe (closed), someone turned it off, often during other work. Only turn it back on if you are comfortable doing so and you do not smell gas.
• Older furnaces with a standing pilot light (mostly units from before the 1990s): look through the viewing window. If the pilot is out, follow the relighting steps printed on the furnace label exactly. If it will not stay lit, the thermocouple is probably worn out, and that is a repair job.

Step 8: Read the blinking light (the error code)

Most furnaces made in the last 25 years have a small status light on the control board. You can usually see it through a little window in the lower door.

• Count the blinks. For example: 3 blinks, a pause, then 3 blinks again.
• Match the count to the chart printed on the inside of the furnace door.
• Take a photo of the light and the chart before you reset anything. A reset clears the code. That photo can save a technician a lot of time.

Common meanings include pressure switch problems, ignition failure (lockout), flame sensor problems, and open limit switch. The exact codes differ by brand.

Step 9: Reset the furnace once

If the furnace has locked out, cutting power resets the control board.

• Turn off the furnace service switch (Step 3).
• Wait 30 to 60 seconds.
• Turn it back on and raise the thermostat.

Only do this once or twice. Lockouts exist to protect you. If the furnace keeps locking out, something is wrong, such as a dirty flame sensor, a failing igniter, or a venting problem. Resetting it over and over can make the problem worse.

Some older furnaces also have a red reset button on the blower motor. Do not press it more than once. If the motor trips again, it needs service.

Step 10: For high-efficiency furnaces, check the drain and the outside pipes

If your furnace has two white plastic (PVC) pipes coming out of it, it is a high-efficiency (condensing) furnace. These have two extra failure points, and both are common in Denver winters.

Blocked intake or exhaust pipes. The pipes usually exit through a side wall or the roof. After a snowstorm, wind-driven snow can bury them. During long cold snaps, frost can build up on the exhaust. When this happens, a safety switch stops the furnace from starting.

• Go outside and find where the two pipes end.
• Gently clear away snow, ice, leaves, or nests.
• Never tape, cover, or block these pipes.

A clogged condensate drain or a full condensate pump. These furnaces make water as they run. If the drain tube or trap clogs, or if the small condensate pump fails, a sensor keeps the furnace off.

• Look for water in the furnace base, a full pump reservoir, or a drain tube packed with slime.
• If the pump is full and not running, check that it is plugged in.

Our furnace service visits include cleaning the condensate drains on high-efficiency furnaces for exactly this reason.

Still no heat? What the problem likely is

If you have done all 10 steps and the furnace still will not turn on, the problem is likely inside the furnace. Here are the most common parts that fail, and what each repair typically involves.

What does furnace repair cost in Denver?

Prices depend on the part, the brand, and how easy the furnace is to reach. As a rough guide for the Denver metro in 2026:

• Minor repairs (cleaning a flame sensor, replacing a fuse or a thermostat part): often under $250
• Mid-range repairs (igniter, pressure switch, capacitor, limit switch): often $200 to $500
• Major repairs (inducer motor, blower motor, control board): often $500 to $1,500 or more
• Heat exchanger failure: often the point where replacement makes more sense than repair

We always tell you the price before we start work.

Repair or replace?

If your furnace is more than 15 years old and needs a major repair, get a replacement quote alongside the repair quote before you decide. Colorado’s ultra-low NOx rule for new gas furnaces took effect January 1, 2026. It does not force you to replace a working furnace, but it has changed which models you can buy and what they cost. Our guides on when to replace your furnace in Denver and the new Colorado furnace law explain the trade-offs.

Why furnaces often fail on the first cold night in Denver

We see a clear pattern every fall across Denver, Arvada, Westminster, Lakewood, and Golden. The first real cold snap brings a wave of no-heat calls. There are a few reasons:

• The furnace sat unused for months. Dust settles on the flame sensor and burners over the summer. A dirty flame sensor is one of the most common reasons a furnace lights and then shuts off.
• Weak parts fail under load. An igniter or motor that was already worn often gives out on the first long run of the season.
• Colorado’s early storms. Denver can get its first real snow in September or October. Heavy, wet snow can bury sidewall vents on high-efficiency furnaces.
• High altitude. At 5,280 feet, furnaces need to be set up correctly for the thinner air. A furnace that was never properly adjusted for altitude can have combustion and ignition problems. Our guide on how Denver’s altitude affects your furnace explains why.

The best way to avoid a no-heat night is a fall furnace tune-up before the heating season starts. A technician cleans the flame sensor, tests the igniter, and checks the safety switches while it is still warm outside.

How to stay safe and warm while you wait for repair

If you are waiting for a technician during a cold snap:

• Close blinds and curtains at night, and open them on south-facing windows during sunny days. Denver’s winter sun can add real warmth.
• Close doors to rooms you are not using, and gather everyone in one or two rooms.
• Use space heaters safely. Plug them straight into a wall outlet (not an extension cord), keep them 3 feet from anything that can burn, and turn them off when you leave the room or go to sleep.
• Never use a gas oven, stove, grill, or generator to heat your home. These can produce deadly carbon monoxide.
• Protect your pipes. If it is well below freezing, open cabinet doors under sinks on outside walls and let faucets drip slowly.

What to tell the technician when you call

You can speed up the repair by sharing:

• The furnace brand and approximate age (it is on the label inside the door)
• Whether it is a standard furnace (metal flue pipe) or high-efficiency (two white PVC pipes)
• What you hear when you turn up the heat: nothing, a click, a fan, or a flame that lights then shuts off
• The blink code photo from Step 8
• What you have already tried from this checklist

Get your heat back today

If your furnace still will not turn on, our NATE-certified technicians can diagnose it quickly. Our trucks carry common parts like igniters, flame sensors, and pressure switches, so many repairs can be done on the first visit.

Comfy Cave Heating & Air is based at 5225 Ingalls Street in Arvada and serves homeowners across Denver and the surrounding metro. Our owner, Von Etzwiler, has worked on furnaces in the Denver area since 2006.

Call 303-645-4889 for furnace repair in Denver, or request service online. If you would rather avoid this problem next winter, ask about our HVAC maintenance plans.

Frequently asked questions