How do I know if my furnace ignition switch is bad?

If you suspect your furnace ignition switch may be faulty, there are some signs you can check for to determine if it needs to be replaced. The ignition switch is an important safety component that allows gas to flow to the burner, so a faulty switch can prevent the furnace from lighting properly.

What Does the Furnace Ignition Switch Do?

The furnace ignition switch, also known as the gas valve or thermocouple, is part of the furnace gas control system. When you turn up your thermostat and call for heat, this signals the ignition switch to open and allow gas to flow to the burners. It also causes a spark to ignite the gas to produce a flame.

Once the gas is lit and the furnace senses the flame, a thermocouple in the ignition switch heats up. This heat keeps the gas valve open so gas continues flowing to the burners. If the pilot light goes out and the thermocouple cools down, the gas valve automatically closes to prevent raw gas from accumulating in the furnace. This helps prevent dangerous gas leaks and possible explosions.

6 Signs Your Furnace Ignition Switch May Be Bad

Here are some of the most common signs of a faulty or failing furnace ignition switch:

1. Furnace Won’t Ignite

If your furnace won’t start up and produce a flame, the ignition switch likely isn’t opening the gas valve as it should. You may hear it clicking rapidly as it tries to light, but with no success. This is one of the most common signs of a bad ignition switch.

2. Pilot Light Keeps Going Out

If the pilot light continues to go out, even after multiple attempts at relighting it, the thermocouple that senses the pilot flame may be malfunctioning. This component tells the gas valve when to stay open. If it’s faulty, the valve shuts off the gas too soon.

3. Burners Turn Off Mid-cycle

Sometimes a faulty thermocouple causes the burners to light initially but then shut off after running for a while. This intermittent operation occurs if the thermocouple isn’t hot enough to keep the gas valve open during the full heating cycle.

4. Soot Buildup on Pilot Assembly

Carbon deposits and soot on the pilot assembly can block the thermocouple from heating up properly. This prevents it from registering the pilot flame, causing the system to shut down. Excess soot indicates improper gas combustion.

5. Control Board Failure Code

If your furnace control board shows an ignition failure code, the problem likely stems from a fault in the ignition components. Error codes like 3 red flashes indicate the system failed to detect or sustain a flame due to ignition switch issues.

6. Burners Have Blue Flames

The gas burners normally produce short yellow flames when operating correctly. Long, blue flames or frequent flaring indicate improper gas combustion and potential problems with the ignition components. Have your furnace serviced right away if you observe this.

Testing the Ignition Switch

To help diagnose ignition issues, there are some basic tests you can do to check your furnace switch:

Continuity Test

Use a multimeter to test for continuity between the two terminals on the gas valve. You should get a reading if the switch is closed and power is being sent. No reading means the switch is open and faulty.

Thermocouple Test

Heat up the thermocouple with a lighter for 30 seconds, then check if the switch closes and sends power. No response means the thermocouple needs replacement.

Voltage Test

Check for 24-30V at the control valve terminals during ignition. No voltage reading indicates a wiring or circuit board issue.

When to Repair or Replace Your Ignition Switch

Here are some guidelines for when to replace your faulty furnace ignition switch:

  • Replace the thermocouple if the pilot light keeps going out.
  • Replace the gas valve if the furnace won’t ignite or burners shut down mid-cycle.
  • Replace the ignition switch if testing shows open contacts or a faulty thermocouple.
  • Repair sooty or corroded pilot assemblies before replacing other components.

Waiting too long to replace a malfunctioning ignition switch can lead to dangerous gas leakage or carbon monoxide poisoning. Most experts recommend replacement after 5-10 years of use.

DIY vs Professional Replacement

Replacing a furnace ignition switch involves disassembling the burner compartment and disconnecting power and gas lines. This poses some safety risks if not done properly:

  • Gas leaks and explosions if lines are disconnected incorrectly.
  • Carbon monoxide leakage if components are reassembled improperly.
  • Electrical shock from mishandled wiring.
  • Damaged components from improper handling.

For these reasons, most HVAC pros recommend calling a licensed technician for ignition switch replacement. They have the necessary skills and expertise to:

  • Safely disconnect and cap off gas lines.
  • Remove and replace old igniter components.
  • Reconnect new ignition switch properly.
  • Test for gas leaks to ensure safety.
  • Clean surrounding furnace parts.
  • Calibrate and test full ignition operation.

That said, it is possible for handy homeowners to replace their own ignition switches by carefully following safety precautions and the manufacturer instructions. Just be extremely cautious when working with gas connections.

Cost to Replace a Furnace Ignition Switch

The costs to replace a faulty furnace ignition switch include:

  • Ignition switch part: $20-$150
  • Labor: $200-$500
  • Service call fee: $50-$100

On average, expect to pay $300 to $600 for a professional ignition switch replacement. The part itself is not too pricey, but labor accounts for most repair costs. You can save money by doing it yourself if you are experienced with HVAC repairs.


How do I light my pilot light if the ignition is bad?

You may be able to manually light your furnace pilot light if the ignition switch fails. Turn the gas valve to “pilot”, then press the reset button while triggering the igniter and hold for 60 seconds after lighting the flame. This bypasses the ignition switch.

Why does my pilot light keep going out after replacing the thermocouple?

This may mean the gas valve or igniter is still faulty. Other causes include a weak pilot flame, lack of gas pressure, or soot buildup preventing the thermocouple from heating properly. Proper cleaning and component testing is needed.

Can I jumpstart my furnace ignition with a wire?

No, you should never try to manually jumpstart or bypass the furnace ignition switch. This over-rides important safety controls. Call a pro for proper diagnosis and repair.

The Bottom Line

A faulty furnace ignition switch prevents proper gas flow and ignition, making repair necessary. Warning signs include failure to ignite, sooty buildup, and mid-cycle burner shut down. Replace switches over 5-10 years old. Hire an HVAC technician for best safety and results when replacing furnace ignition components.