How To Test Your Furnace’s Ignition Control Module

What is an Ignition Control Module?

The Ignition Control Module (ICM) is a part of your furnace with one primary function: to light the burners. It receives a signal from the thermostat to start. It sparks (direct spark ignition) and sends power to the gas valve. It shuts down if the Ignition Control Module sensor doesn’t recognize a flame within a few seconds.

Unlike a fan board or full control circuit board, an ignition control module does not control the blower. It only controls lighting the burners. Because it uses an electronically-controlled ignition to light the burners, it is more energy efficient, generally safer, and more reliable than the standing pilot light found in most older gas furnace models.

However, ignition control modules can still encounter problems. Electronic ignition failure is quite common and can prevent your furnace from turning on.

Some furnaces utilize Intermittent Pilot (IP) systems, where the pilot only ignites the burner gas when the thermostat demands heat. Others use Hot Surface Ignitors (HSI), where electricity passes through a filament and heats up, eventually igniting the gas for the burner.

Could My Furnace Ignitor Be Bad?

Several things can happen when you have a faulty furnace ignition control module, but the obvious symptom is usually no heat. This is because heating doesn’t happen without furnace ignition. The ignitor must light the burners to produce warm air.

There may still be air coming out of the ducts; it just won’t be warm. This can be due to the circulator blower running as a safety measure when the furnace fails to light properly.

With an intermittent pilot system, your ignition may not spark at all, or the spark happens but does not light the pilot. Sometimes, the pilot will light, but the gas valve may not open, and the burner will not light. HSIs can also malfunction over time due to normal wear, damage due to improper handling, or a too-high electrical current in your home. Your furnace may start short cycling as the igniter attempts to light the furnace again and again. You may also notice the ignitor isn’t glowing red.

Troubleshooting Your Ignition Control Module

Many things can happen during the ignition process that can cause the system to fail. From failure to light the pilot to faulty sensors, any issue will signal to the circuit board that it isn’t safe to start the furnace. A few simple ways to investigate are:

  • Turn the furnace off, then wait a few minutes before turning it back on to see if the issue is resolved.
  • Check your furnace filter. A dirty filter can cause your furnace ignition to cycle on and off repeatedly, leading to malfunction.
  • Dirty filters, clogged ductwork, or plugged flue can cause limit switches to open when excessive heat buildup occurs.
  • Look at the ignition cable to ensure that there are no cracks or breaks and the connections are clean and secure.

How to Test for Basic Ignition Issues in the Furnace

To test your furnace’s ability to ignite, you’ll need a multimeter, but first, you must be sure you are taking all safety precautions.

  • Turn off all power to the furnace and close off the gas. Allow your furnace to cool before attempting any testing.
  • Remove the panel door and locate the Ignition Control Module (usually positioned near the gas port). If you see visible damage, then it needs to be replaced. If not, detach the wires, loosen the screw, and remove the Ignition Control Module from its socket.
  • To check if the ignition module is getting voltage, set the multimeter to AC voltage (VAC) reading and check for voltage at the thermostat input or 24VAC input to the Ignition Control Module. You should get a reading of 24VAC with respect to common or ground. If the multimeter stays at zero, a technician may be needed to troubleshoot the furnace for loss of power.
  • To check for ignitor resistance, set the multimeter to resistance readings (ohms). Remove the igniter from the circuit, connect the meter probes to the ignitor, and check the reading. A correctly functioning ignitor will give a resistance value between 40 – 200 ohms. If your ignitor has lower resistance, it’s probably ready to be replaced.

Replacement Ignition Control Modules

ICM Controls manufactures top-quality, cost-effective ignition control modules for gas and oil boilers, furnaces, and other heating appliances. Visit ICM’s website to cross-reference the part you need if you’re comfortable installing it yourself. If not, call an HVAC expert in your area.