Furnace Not Turning On — Step-by-Step Diagnosis

A furnace that will not turn on is almost always caused by one of five things: a tripped circuit breaker, a clogged filter triggering a safety shutoff, a thermostat issue, a failed igniter, or a fault code lockout. This guide addresses them in order from simplest to fix to most involved.

1. 1

   Step 1: Check the thermostat settings and batteries

   Confirm the thermostat is set to Heat, not Cool or Fan Only, and the set temperature is at least 3°F above the current room temperature. Replace the batteries in battery-powered thermostats — a low battery causes intermittent failure that looks like a furnace problem. If the display is blank, the thermostat has lost power.

2. 2

   Step 2: Check the furnace switch and circuit breaker

   The furnace has a dedicated on/off switch on or near the unit that looks like a light switch — it is sometimes accidentally turned off. Confirm it is in the ON position. Then check the electrical panel for a tripped breaker labeled Furnace or HVAC. Reset it once. If it trips again immediately, stop and call an electrician.

3. 3

   Step 3: Replace the air filter

   A severely clogged filter causes the furnace to overheat and triggers the high-limit safety switch, shutting the furnace down as a protective measure. Replace the filter. After replacing, wait 30 minutes for the furnace to cool before trying to restart. If the furnace starts and then shuts off again within minutes, the high-limit switch may have failed and need replacement.

4. 4

   Step 4: Look for a fault code on the furnace

   Most furnaces manufactured after 1995 have a diagnostic LED that flashes a fault code when there is a problem. The code legend is printed on a label inside the furnace door. Count the flashes (e.g. 3 short, 1 long = code 31). Common codes include: igniter failure, pressure switch stuck, rollout switch open, and limit switch open. Document the code before proceeding.

5. 5

   Step 5: Inspect and test the igniter

   The hot surface igniter (a fragile ceramic glow bar) is the most commonly replaced furnace component. With power off, locate the igniter near the burners — it looks like a small white or grey rectangular element. Inspect it visually for cracks or breaks. Test continuity with a multimeter set to Ohms — a good igniter reads 40–200 Ω; infinite resistance means it is broken. Order a replacement igniter matched to your furnace model number.