A gas furnace can’t work without a flame and combustion, which can not happen without a properly functioning control module. In this article, you’ll learn more about the ignition control module, how it works, what it does and how to recognize symptoms when the ignition control module unit starts to fail.
What the ignition control module is
You can think of an ignition control module as a central hub that distributes power to all the electrical components in your furnace system. The inducer motor, ignitor, gas valve and other systems all run on electrical power that’s regulated and switched on and off by the ignition control module.
Many older systems feature an ignition control board, which manages the ignitor, flame sensing circuit and the valve. This system is less common now, as it’s been replaced by integrated furnace control designs. In an integrated furnace control, the blower motor is also managed along with the ignition controls. It’s a system that’s more modern, safer and more reliable than older units that had a constantly-burning pilot light flame.
Furnace ignition control module troubleshooting
These can all be symptoms to tell if a furnace ignition control module is bad:
- Warning lights: On modern units, furnace ignition control module problems are made easier to track down with self-diagnosis. Not unlike the Check Engine light on a vehicle, failing HVAC components will trigger a red warning light on the control board itself, pointing a tech in the right direction for troubleshooting. On some systems, the LEDs might be programmed to blink in a certain sequence or a certain number of times, indicating an error code that you can then interpret by comparing to the furnace manual or schematic (the schematic can sometimes be found behind the lower access panel).
- Poor temperature management: When there doesn’t seem to be a problem related to the thermostat but the home is uncomfortably cool or hot, there’s a chance that the furnace ignition control module is the culprit.
- Interruptions in event sequence: A furnace control board is programmed to run the ignition process in a certain series of steps. A typical sequence might be: induced draft motor starts/pressure switch closes/ignitor surface heats up/spark is delivered/gas valve opens/furnace lights/flame sensor detects flame. If (for instance) the ignition doesn’t turn on or the blower continues to run after everything else has stopped, these can be bad ignition control module symptoms.
What can cause an ignition control module to fail?
These are all some potential causes of a failed ignition control module:
- Loose or corroded wiring or terminals: HVAC units and ignition control modules run in a difficult environment. The vibration from a blower motor can be enough to cause wires to come loose, resulting in a short circuit and the failure of the ignition control board for the unit. At least this is a problem that will be pretty apparent on inspection and should be easy to remedy.
- Clogged furnace filters: A saturated filter can impede airflow to a point where the blower motor overheats, causing damage to the whole system.
- Dirty or wrong fuse: Replacing the system’s fuse with one that’s not the right value can cause the circuit board to break down, as can a fuse and fuse holder that have become dirty with dust and fuzz.
- Limit switch or pressure switch stuck in open position: Either one of these switches being open will prevent the board from delivering a signal to the device or system that it’s designed to control. In these cases, you’ll need to start looking for the problem that resulted in these switches being open rather than closed.
Steps for diagnosis and repair
Here’s a broad set of steps for furnace ignition control module troubleshooting and repair.
- Check furnace diagnostic LEDs and reference available documentation.
- Remove access panels (usually either held down with latches or screws).
- Tape down the door switch; many systems have a switch which automatically pops up when the door is opened, opening a circuit. Taping the switch down will “fool” the system into thinking the door is closed.
- Check the control board indicator light. The blinking LED on the board is what indicates that power is reaching it. If no indicator light is present, it’s time to start checking every connection along the power flow to the board.
- Locate the common wire on the transformer. The transformer will be a small rectangular box that’s attached to the furnace with screws, and will have four wires: two high voltage and two low voltage. Of the two high voltage wires, the one with the black insulation will be the common wire.
- Locate the line voltage, which will be carried by the wire that runs from the door switch to the board and is usually marked “LINE.” Using a multimeter, check the voltage from the terminal of the line voltage to the common wire on the transformer; this should read 120 volts.
- Test power at the transformer, using your multimeter. The high voltage side should be 120 volts and the low voltage side 24 volts. Remember that a transformer is designed to either raise or reduce voltage; if you don’t see 24 volts on the low voltage side, the transformer is faulty and should be replaced.
- If you see 24 volts from the low voltage side of the transformer, trace the low voltage wires to the board where they should connect at a Molex plug. Place your meter leads on the Molex plug and you should see 24 volts again.
- If everything still looks good, locate the terminal strip in the furnace’s controls (usually at the edge of the control board). The terminal strip will have five wires of different colors and a set of letters to show which wire goes where (R, W, Y, G, C). The R wire is for low voltage, and the C wire is for the common circuit; test these two with your meter’s probes and once again you should see 24 volts.
And that’s about it! If everything in this set of procedures checks out, you can rule out a problem with the furnace ignition control module. If at some point you stop seeing the proper voltages on the meter, that would point to either a connection problem or a problem with the module itself.
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