Furnace Not Heating: Thermostat, Pilot, and Filter Fixes

Quick Answer

A furnace not heating usually indicates a tripped breaker, dirty air filter, or faulty thermostat setting. The most common fix is replacing a clogged filter to restore proper airflow and system operation.

A furnace that runs but produces no heat is among the most common winter service calls HVAC technicians handle. In most cases the culprit is a cracked hot surface igniter, an oxide-coated flame sensor rod, or a high-limit switch trip caused by a clogged air filter. Identifying which stage of the ignition sequence failed lets homeowners decide between a five-minute DIY fix and scheduling professional service.

GenericFurnaceSeverity: moderateDifficulty: intermediate75% DIY Success

Time to Fix

15–90 min

Difficulty

intermediate

Parts Cost

$0 (no parts needed)

Tools Needed

Phillips #2 screwdriver, Flathead screwdriver for panel clips and wire terminals

Furnace Not Heating: Thermostat, Pilot, and Filter Fixes

In my experience, nine out of ten furnaces that stop heating have one of three culprits: a clogged filter that tripped the high-limit switch, a cracked hot surface igniter that cannot reach ignition temperature, or a flame sensor coated with oxide buildup that shuts gas off after 2-3 seconds. Filter replacement and flame sensor cleaning are straightforward DIY repairs costing under $10. A replacement igniter runs $15-40 in parts. Gas valve or control board failures are rarer but push costs toward $200-500.

Common Causes

Dirty or completely clogged air filter blocking airflow across the heat exchanger, causing the high-limit thermal switch to trip and shut off burners to prevent overheating.
Cracked or burned-out hot surface igniter, a brittle silicon carbide or silicon nitride element that fails to reach the 2500°F surface temperature needed to ignite the gas.
Flame sensor rod coated with a thin white or grey oxide layer that prevents the control board from detecting an established flame, causing the gas valve to close after 2-5 seconds.
Thermostat set to COOL or FAN-only mode, dead batteries in a wireless thermostat, or a loose low-voltage wire at the R, W, or C terminals on the furnace control board.
Failed draft inducer motor or a cracked rubber hose between the inducer housing and the pressure switch, keeping the pressure switch open and blocking the ignition sequence entirely.
Gas valve coil not receiving the 24VAC signal from the control board due to a failed transformer, blown fuse on the control board, or a failed valve coil itself.

Symptoms You May Notice

Furnace turns on and the inducer motor runs, you hear clicking or see the igniter glow orange, but no flame ignites and the unit shuts down after 3-5 attempts.
Furnace ignites briefly for 2-5 seconds, the flame goes out, and the system cycles this sequence 3 times before going into hard lockout with a flashing LED error code.
Blower fan runs continuously and circulates room-temperature or cold air because the burners never ignited during the heating cycle.
Thermostat calls for heat but no sounds come from the furnace at all, indicating a wiring, transformer, or blown control board fuse upstream of the ignition sequence.
Furnace runs normally for 10-15 minutes then shuts off mid-cycle and struggles to restart, pointing to a high-limit switch trip from restricted airflow over the heat exchanger.

Can you reset a Generic furnace to clear the NOT-HEATING code?

To reset a furnace in hard lockout, set the thermostat to OFF. Find the furnace power switch, typically a wall switch near the unit that looks like a light switch, and turn it OFF. Wait a full 30 seconds, then turn the power switch back ON and set the thermostat to HEAT with the setpoint raised above room temperature. The furnace will attempt a fresh ignition sequence. If lockout returns within one heating cycle, do not keep resetting. The control board is protecting a real fault and a component needs to be diagnosed and replaced.

Tools Required for Diagnosis

Phillips #2 screwdriverFlathead screwdriver for panel clips and wire terminalsDigital multimeter with ohms and AC voltage modesFine steel wool or 220-grit emery cloth for flame sensor cleaningFlashlight or headlampReplacement air filter in the correct dimensions for your unitNeedle-nose pliers for spade wire terminalsReplacement hot surface igniter, model-specific, $15-40

Service / Diagnostic Mode

Most gas furnaces do not have a button-entry diagnostic mode. Instead, locate the small LED status light on the control board, visible through the lower cabinet sight glass or by removing the lower access panel. Watch the LED for a full 30-60 seconds and count the flashes between pauses. The flash code is decoded on a label affixed to the inside of the furnace cabinet door. Common patterns: 3 flashes indicates a pressure switch fault, 4 flashes indicates an open high-limit switch, 5 flashes indicates flame sensed without a call for heat. Write down the code before proceeding with diagnostics.

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test

Expected Range40–200 ohms

ConditionIf Open (OL) or infinite, replace component.

Frequently Asked Questions

Why does my furnace turn on but blow cold air?

When the blower runs but the air is cold, the furnace completed its fan startup but failed to ignite the burners. The control board detected no flame signal and shut off the gas valve, but the blower continued through its normal timed cycle. This is almost always a flame sensor, igniter, or gas supply issue. Check the control board flash code first. If the igniter glows visibly orange for 5-10 seconds but nothing lights, the gas valve, supply shutoff, or gas pressure is the next thing to investigate. A technician can test gas pressure at the manifold with a manometer.

How much does it cost to repair a furnace that is not heating?

Repair cost depends on which component failed. A flame sensor cleaning is free as a DIY fix or $75-100 as a service call. A replacement hot surface igniter is $15-40 in parts and $100-150 with labor. A new pressure switch runs $20-60 in parts. A draft inducer motor replacement costs $150-400 installed. A gas valve runs $100-250 in parts plus labor. A control board replacement is the most expensive common repair at $200-500 installed. Most furnace no-heat repairs that require a technician fall in the $150-350 range when the cause is a single failed component.

How do I know if my furnace flame sensor is bad?

The classic symptom of a failing flame sensor is a furnace that lights briefly, burns for 2-5 seconds, then shuts off and retries 2-3 more times before locking out. You can often see the burners ignite through the sight glass and then go dark. The flame sensor rod itself will show a white, chalky, or grey oxide coating rather than bare shiny metal. Cleaning it with steel wool takes about 10 minutes and resolves the issue roughly 40-50% of the time. If cleaning does not help after one test cycle, the sensor rod may be cracked or the control board input circuit has failed and needs replacement.

Can a dirty air filter really stop a furnace from heating?

Yes, and it is one of the most frequently overlooked causes I see on service calls. When the filter is severely clogged, airflow over the heat exchanger drops and the exchanger surface temperature rises rapidly. The high-limit switch, a thermal cutout mounted directly on or near the heat exchanger, trips and shuts the burners off to prevent cracking the exchanger. Most high-limit switches auto-reset once the exchanger cools, so the furnace may attempt to restart and trip again in a loop. Replacing the filter and waiting 20-30 minutes for the high-limit to cool and reset is all it takes. Replace filters every 60-90 days.

Is it safe to reset my furnace lockout by myself?

One manual reset is generally safe to perform. Turn the furnace power switch off for 30 seconds, then back on. If the furnace runs a full heating cycle without locking out again, you are likely fine. The concern is performing repeated resets: if the lockout returns in the same day, the control board is detecting a real fault, such as an open flame rollout switch or a failed inducer, and resetting it repeatedly will not fix the underlying problem. It may also mask a genuine safety condition. If a furnace locks out twice in the same day after a reset, stop resetting and call a licensed HVAC technician.

When should I replace my furnace instead of repairing it?

A well-maintained gas furnace typically lasts 18-25 years. The standard industry rule of thumb is to replace when the repair cost exceeds 50% of the cost of a new unit, or when the furnace is over 15-18 years old and facing its first major repair. A new mid-efficiency furnace at 80 AFUE costs roughly $1,500-3,500 installed. A high-efficiency 95-plus AFUE unit runs $2,500-5,000 installed. If your furnace is 20 years old and facing a $400-500 heat exchanger repair, replacement is usually the smarter financial decision, especially given the lower operating costs and improved reliability of modern variable-speed equipment.

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 17, 2026