When the furnace won’t start, run checks, verify power and thermostat, clear filter and drains, then review ignition before calling a technician.
Safety First Before Any Fix
Start with safety. Now if you smell gas or a fuel alarm sounds, leave the home and call emergency services from outside. See the NFPA guidance on gas leaks for the exact steps. Keep a working CO alarm on each level and near sleeping areas; the EPA page on CO alarms summarizes the basics.
Furnace Not Starting: Calm, Step-By-Step Checks
Most no-heat calls trace back to simple items: a thermostat setting, a switch, a tripped breaker, a clogged filter, or a full condensate trap. Work through the list below in order. You’ll spot the fix faster and avoid repeat lockouts.
Quick Reference Table: What To Check And What It Means
| Check | Where | What “OK” Looks Like |
|---|---|---|
| Thermostat mode & setpoint | Wall control | Mode on Heat; setpoint above room temp; fan on Auto |
| Furnace service switch | Switch near unit | Toggle up/on; light switch style, often red plate |
| Breaker or fuse | Main panel | Breaker not tripped; fuse intact; label matches furnace |
| Front door interlock | Blower panel | Panel seated tight; latch fully closed |
| Air filter | Return grille or blower slot | Clean, sized correctly, arrow toward furnace |
| Condensate line/trap | White PVC at furnace | No standing water; line not frozen or kinked |
| Intake/exhaust terminations | Outside wall or roof | Open, no ice, leaves, or nests |
| Call for heat lights/code | Furnace control window | Steady status or normal blink pattern |
Thermostat And Power: Small Things That Stop Heat
Set the thermostat to Heat and raise the setpoint five degrees above room temperature. If the screen is blank, replace the batteries or reseat the common wire. Next, flip the dedicated furnace service switch off and back on. Then check the panel for a tripped breaker and reset it once. If it trips again, stop and schedule a pro.
Many units have a door safety switch. If the blower panel isn’t tight, the switch stays open and the furnace sits idle. Reseat the panel so the latch clicks. If there’s a small round view window, note any flash codes and jot them down.
Smart Thermostat Setup Quirks
Smart stats can block heat if the system type is set wrong. In the app or device menu, confirm it’s set to a gas furnace, not a heat pump. If the stat needs a common wire and it’s missing, the device may reboot or drop out during a heat call. Run new wire or add a listed power kit instead of borrowing from a safety circuit.
Reading Blink Codes Without Guesswork
Most boards flash a two-digit code during faults. The legend often sits on the back of the blower door or on a sticker near the window. Count the long blinks as tens and the short as ones. Snap a photo before you reset power so you can share the code with a technician if needed.
Reset Timing That Won’t Fight The Board
After three to five failed tries, many boards wait several minutes before retrying. If you cut power too quickly, the board may never clear the wait timer. Let the lights stop, give it a full minute with power off, then restore power and watch a single ignition sequence. Repeated flips slow you down and can hide the real cause.
Airflow And Filters: Keep The Safety Switch Happy
A packed filter starves the heat exchanger, the limit switch opens, and the board locks out to protect the system. Slide the filter out and check the size printed on the frame. Replace with the same size, pointing the arrow toward the furnace. If you run a high MERV filter and see short heat cycles, try a fresh one or step down one rating to reduce resistance.
Blocked supply or return grilles also cut airflow. Open floor registers, clear rugs, and check large furniture. In the basement or a closet, keep boxes away from the return side so air can move freely.
Condensate And Vents: Hidden Water And Blocked Pipes
High-efficiency models make water as they pull more heat from the flue gases. That water drains through a small trap and line. If the trap clogs or a float switch senses a backup, the board stops the heat call. Look for a clear vinyl or white PVC line leaving the cabinet and verify a steady drip during a heat cycle. In cold snaps, outside runs can freeze; thaw gently and insulate later.
When the line is sludged with algae, a wet/dry vacuum on the outside discharge can pull the blockage. Remove the trap only if the manual shows a simple push-fit design and you’re comfortable reseating it. Refill the trap with clean water, then restart power to clear any fault.
Ignition Basics: Pilot, Hot Surface, Or Spark
Gas furnaces light burners in one of three common ways. The steps you take depend on the type on your model. If the data plate lists “intermittent pilot,” you’ll see a small pilot flame that lights only during a call. “Hot surface igniter” units glow orange before the burners light. “Direct spark” units skip the glow and tick like a grill ignition.
Flame Sensor And Lockouts: Burner Lights Then Dies
If burners light for a few seconds and go out, the flame sensor may be dirty. The sensor reads a small current through the flame; soot blocks it. After cutting power, remove the thin rod with one screw and wipe gently with a clean, fine abrasive pad. Do not sand the wire or bend the rod. Reinstall, restore power, and call for heat once. If it still trips, stop resets and schedule service.
Modern boards shut down after several failed tries to prevent raw gas. Clearing power for a minute often resets the timer. Avoid rapid on-off cycling, which can lock the board again.
Pressure Switch, Draft, And Intake: The Combustion Path
The pressure switch proves that the inducer fan is moving the right amount of air through the heat exchanger. If the switch doesn’t close, the board won’t allow ignition. Start outside: clear snow, lint, leaves, or nests from the intake and exhaust. Inside, listen for the inducer starting. If it hums but won’t spin, or the switch clicks open and closed, that’s a service visit.
Water in the draft tubing can also hold the switch open. Check for low sags in the small rubber tube and drain them. Reattach any loose hose so it sits in a smooth rise without kinks.
Ignition Types And What You Can Do
| Ignition Type | Symptom | Homeowner Action |
|---|---|---|
| Standing pilot (older) | No flame at pilot | Follow the lighting label; hold reset per label; if flame won’t stay, stop |
| Intermittent pilot | Pilot lights, burners don’t | Watch once through glass; if it retries then locks out, call service |
| Hot surface igniter | No orange glow | Cycle power; if still no glow, leave repair to a technician |
| Direct spark | Ticks but no flame | Check intake/exhaust for blockage; avoid repeated resets |
Gas Supply And Valves: What You Can Safely Check
Set the appliance gas shutoff handle in line with the pipe. If the handle is across the pipe, the valve is closed. Never move a sticky valve; leave that for a pro. If you recently had gas service, the meter may include a shutoff that was left closed.
If you smell gas at any time, stop, leave the home, and call the utility or 911 from outside. Do not cycle switches or use a phone inside while a leak is suspected.
Electrical Signs You Shouldn’t Ignore
Burn marks, melted insulation, or repeated breaker trips point to faults that need a licensed technician. So does a loud buzz from the transformer, a blower that never stops, or a control board with burnt tracks. Power down at the switch and the breaker and leave it off until checked.
When To Call A Pro
Call in a technician for gas valve work, control board faults, inducer problems, cracked or broken PVC vents, persistent igniter failures, or any repeated lockout. Warranty terms can be affected by DIY repairs, so review the manual before replacing parts.
Preventive Moves That Keep Heat Coming
Change filters on a schedule that matches your home: every one to three months for 1-inch filters, three to six months for deep media, sooner with pets or renovations. Keep the area around the furnace clear. Vacuum dust from the burner compartment gently. Verify the condensate line drains freely each fall and insulate any outside run. Test CO alarms monthly, swap batteries yearly, and replace alarms per the label.
Once a year, book a tune-up. A good visit includes checking heat exchanger condition, confirming gas pressure, testing safety switches, cleaning the flame sensor, checking draft and CO, and verifying temperature rise across the furnace matches the rating plate. Ask for a written report so you can track readings over time.
Quick Fix Scenarios
Furnace clicks but won’t fire? Clear intake and exhaust, check the filter, then try one power reset. If it still won’t fire, stop retries.
Blower runs but air is cold? Heat may be locked out. Cycle power once and watch the ignition sequence through the window. Note any code.
Unit starts then shuts off in a minute? Swap the filter and open registers. If the limit switch opened, airflow fixes often help.
Thermostat says heating but nothing happens? Check the service switch, panel door, and breaker. Replace thermostat batteries.