Roll Out Switch Diagnostic
Test your furnace roll out switches to ensure they are working normally.
Start here
Roll out switches are a critical safety feature of mid- and high-efficiency furnaces. Roll out switches ‘pop’ open when the temperature near the furnace burner box exceeds a safe threshold, which prevents the furnace from turning on or running. Unlike limit switches, roll out switches must be manually reset after they pop open.
Refer to the furnace glossary if you need help identifying furnace parts or terms.
Roll out switch failure is typically caused by a faulty inducer motor, a blocked intake or exhaust vent, or a partial blockage of the secondary heat exchanger.
Never bypass a limit or roll out switch under any circumstance.
Bypassing a limit or roll out switch can cause your furnace to overheat and catch fire, causing injury and death.
Testing the roll out switches
Use this procedure to diagnose if a faulty roll out switch is the source of your furnace problem — after you’ve done basic troubleshooting.
Tools needed
- multimeter that can measure continuity or resistance (ohms)
Testing procedure
- Turn off the power to the furnace.
- Remove upper furnace cabinet and locate the roll out switches. They are located on each side of the burner box and are round, about the diameter of a nickel. There are two wires leading to each switch.
- Use your finger to feel for a small nub on the backside of the roll out switch, between where the two wires connect.
- Gently push the nub inwards, toward the roll out switch. You may feel it ‘pop’ in. This is how to manually reset the roll out switch.
- Repeat the process for the roll out switch on the other side of burner box.
- If you reset one or both of the roll out switches, continue to Interpreting results below.
- If you did not reset either switch, remove the wires connecting to each roll out switch.
- With wires removed, test both roll out switches for continuity / resistance and make note of the results.
- Reconnect the wires to the roll out switches and reassemble the furnace.
Interpreting results
Determine which of the following cases matches your result.
If you reset one or both roll out switches
If you ‘reset’ a roll out switch, reassemble the furnace, turn the power to the furnace back on, set the thermostat to call for heat, and see if it starts. If the furnace starts up, it confirms that the rollout switch popped and caused furnace shutdown.
While it’s good that your furnace is running again, a popped roll out switch is almost always the symptom of another furnace issue.
The first possible issue is a blocked intake or exhaust vent, usually caused by snow or frost on extremely cold days (-20C and below). Check outside and ensure that these are clear of obstruction for the entirety of the length exposed to outdoor temperatures. Refer to the furnace maintenance guide.
The second possible issue is a failed inducer motor, which will usually cause other issues with the furnace, and be noisy enough to notice. If you suspect an inducer motor problem, you can test it using the inducer motor diagnostic.
The third possible issue is a partial blockage of the secondary heat exchanger. When blocked, flames can ‘roll out’ from the heat exchanger, causing increased temperatures around the burner box. In this case, the roll out switch will ‘pop’ open again. Usually after a few hours or days of heavy operation. Resetting the roll out switch will provide temporary restoration of heat, but you should call a professional technician to diagnose the furnace ASAP.
If you did not reset either roll out switch and tested for continuity / resistance
If there was no continuity or you measured resistance across the roll out switch connectors (and the nub on back is fully seated and reset), that roll out switch needs to be replaced.
If there is continuity or you measured no resistance across the roll out switch connectors, there is no issue with that roll out switch.
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Next steps
If, after confirming the rollout switches are working normally, you reassemble the furnace but continue to get open limit or roll out switch errors, continue to the limit switch diagnostic.
If, after confirming that the limit switches are also working normally, you continue to get open limit or roll out switch errors, the furnace control board may be faulty and need replacement. At this point, you should call a professional technician to diagnose the furnace.
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This website is for informational purposes only.
Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.