Heating Troubleshooting

Heater Not Turning On?

10 common causes and DIY fixes for when your heater, furnace, or heat pump won't start. Most fixes take under 15 minutes and cost nothing.

Last updated: February 2026

Safety Warning

If you smell gas (rotten egg odor), leave the house immediately. Do not flip any switches or use your phone inside. Call your gas company from outside. For all other troubleshooting, turn off the furnace breaker before inspecting wiring or internal components.

Quick Diagnosis: What's Your Symptom?

1

Thermostat screen is blank

Replace batteries or check breaker → Step 2

2

Thermostat is on but heater does nothing

Check mode setting + W wire connection → Step 1 & 8

3

Heater starts then shuts off in seconds

Clean flame sensor or replace air filter → Step 7

4

AC works but heat doesn't

Check furnace ignition + flame sensor → Step 5-7

5

Heater blows cold air only

Pilot light out or igniter failed → Step 6

6

Furnace makes clicking but won't ignite

Gas valve issue or failed igniter → Call a pro

7

Heater cycles on and off repeatedly

Dirty filter or oversized unit → Step 4

10-Step Troubleshooting Guide

1

Check the thermostat settings

Make sure your thermostat is set to "Heat" mode (not "Cool," "Off," or "Auto"). Set the target temperature at least 3-5 degrees above the current room temperature — most thermostats won't trigger the furnace for a 1-2 degree difference. If you have a programmable thermostat, check that the schedule hasn't set it to a lower temperature. Press "Hold" or "Override" to manually set the temperature.

2

Replace the thermostat batteries

Dead or weak batteries are the single most common reason a heater won't turn on. Most Honeywell, White-Rodgers, and Emerson thermostats use 2x AA or 2x AAA batteries. If the display is dim, blank, or flashing a low battery icon, replace them immediately. Even if the display looks fine, batteries below 2.4V can't reliably send the heat signal. Use fresh alkaline batteries — not rechargeable.

3

Check the circuit breaker and power switches

Your heating system typically has TWO power shutoffs: (1) the circuit breaker in your electrical panel (may be labeled "Furnace," "HVAC," or "Air Handler"), and (2) a dedicated power switch on or near the furnace (looks like a regular light switch, often on the wall or ceiling near the unit). Both must be ON. Flip the breaker fully OFF, wait 30 seconds, and flip it back ON. Check the furnace switch too — it gets accidentally bumped off during maintenance.

4

Replace the air filter

A clogged air filter restricts airflow and can cause the furnace high-limit switch to trip, shutting down the heater as a safety measure. Pull out your filter and hold it up to a light — if you can't see light through it, it's too dirty. Replace with the same size (printed on the filter frame). A clean filter fixes "heater keeps shutting off" problems about 40% of the time. Standard filters cost $5-15 and should be replaced every 1-3 months.

5

Inspect the furnace error code

Most modern furnaces have a small LED window on the control board (visible through the lower access panel). The LED blinks a pattern that indicates the specific problem. Common codes: 1 blink = normal operation, 2 blinks = external lockout (pressure switch or flame sensor), 3 blinks = pressure switch stuck open, 4 blinks = high-limit switch open (airflow problem). Check your furnace manual for the exact code chart — it's usually printed on a sticker inside the access panel.

6

Relight the pilot light (gas furnaces)

Older gas furnaces with standing pilot lights need to be manually relit if the flame goes out. Find the gas valve near the bottom of the furnace — it has a knob with "Off," "Pilot," and "On" positions. Turn to "Off," wait 5 minutes for any residual gas to clear. Turn to "Pilot," press and hold the knob, then use a long lighter to ignite the pilot. Hold the knob for 30-60 seconds after the pilot lights, then release and turn to "On." If the pilot won't stay lit, the thermocouple likely needs replacing ($10-20 part).

7

Clean the flame sensor

This is the #1 professional repair call for furnaces that start but shut off within 5-10 seconds. The flame sensor is a thin metal rod (about 2-3 inches long) located in the burner assembly. Turn off power and gas. Remove the single screw holding the sensor. Gently clean the rod with fine-grit sandpaper (120-grit) or steel wool until it's shiny. Reinstall. This $0 fix saves you a $150-250 service call and takes 10 minutes.

8

Verify the thermostat wiring

Turn off the HVAC breaker. Remove the thermostat from the wall plate. Check that the W wire (white, heat) is securely connected to the W terminal. Also verify the R wire (red, power) is connected. If wires are loose, push them firmly into the terminal and tighten. If you have a heat pump, heat runs through the O/B terminal instead of W. Check our wiring guide for your specific setup.

9

Test the furnace directly (bypass thermostat)

At the thermostat wall plate (with thermostat removed), use a small piece of wire to briefly connect the R terminal to the W terminal. This simulates a heat call. If the furnace fires up, your thermostat is the problem — replace it. If the furnace still doesn't respond, the issue is in the furnace itself (control board, gas valve, or ignition system) and you'll likely need a professional.

10

Check the condensate drain (high-efficiency furnaces)

High-efficiency (90%+) furnaces produce condensation that drains through a small PVC pipe. If this drain line gets clogged with algae or sediment, a safety switch shuts down the furnace. Find the drain line (white PVC pipe, usually 3/4 inch) and pour a cup of white vinegar through it. If water flows freely, the drain is clear. If it backs up, disconnect the line and clear the blockage with a pipe cleaner or compressed air.

If these steps resolved your issue, your thermostat should now be working correctly.

Pro Tip

The "thermostat test" in Step 9 is the most powerful diagnostic you can do. If jumping R to W fires the furnace, you know 100% the problem is the thermostat — not the furnace. This one test saves most homeowners a $150 service call.

Fixes by Heating System Type

🔥 Gas Furnace (Forced Air)

  • Check pilot light or hot surface igniter — if the igniter doesn't glow orange, it's burned out ($15-40 part)
  • Clean flame sensor with fine sandpaper — the #1 cause of "starts then stops" on gas furnaces
  • Look for error code blinking LED on the furnace control board
  • Verify gas supply — check that the gas shutoff valve near the furnace is open (handle parallel to pipe = open)

♨️ Heat Pump

  • Check the outdoor unit — is it running? Clear any ice, snow, or debris blocking the coils
  • Verify the thermostat has an O/B wire connected (reversing valve — required for heat mode)
  • Heat pumps struggle below 30°F — "Emergency Heat" mode bypasses the heat pump and uses backup electric strips
  • If the outdoor unit fan runs but compressor doesn't, the capacitor or compressor may have failed

⚡ Electric Furnace / Baseboard

  • Check the dedicated breaker (electric furnaces use 240V — look for a double-pole breaker)
  • Electric furnaces have sequencers that turn on heating elements in stages — a failed sequencer means partial or no heat
  • Baseboard heaters have individual thermostats — check that each zone's thermostat is turned up
  • Look for a reset button on the unit — high-limit switches trip on electric heaters when airflow is blocked

💧 Boiler / Radiant Heat

  • Check water pressure gauge — most boilers need 12-15 PSI to operate. If below 10 PSI, the low-pressure cutoff stops the boiler
  • Bleed radiators if some rooms heat but others don't — trapped air prevents hot water circulation
  • Check the circulator pump — put your hand on it while the boiler is running. It should be warm and vibrating slightly
  • Verify the zone valve for the affected area is working — you should hear it click when the thermostat calls for heat

Pro Tip

If your thermostat is more than 10 years old and you're having repeated heating issues, consider upgrading to a smart thermostat. Modern models like the Nest or Ecobee can alert you to furnace problems via your phone — before your house gets cold. See our replacement cost guide for pricing.

When to Call a Professional

Call an HVAC technician if:

  • You smell gas anywhere near the furnace
  • The furnace error code indicates a cracked heat exchanger (this is a carbon monoxide risk)
  • You've tried all 10 steps above and the heater still won't start
  • The furnace makes loud banging, screeching, or rattling noises
  • Your carbon monoxide detector is going off
  • The heater is less than 5 years old and under warranty — DIY repairs may void coverage

Average HVAC service call: $75-150 diagnostic fee + parts and labor. Get at least 2 quotes.

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Written by

ThermostatFixer Editorial Team

Our team of HVAC enthusiasts and DIY experts creates detailed thermostat troubleshooting guides, wiring diagrams, and repair tips to help homeowners fix common thermostat issues without calling a technician.