ICP Furnace Showing 80% Limit Fault
December 8, 2025
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Most gas furnaces have several safety limits, main limits, roll-outs, 80%’s will have a DSS (Daft Safety Switch) These safety
With the heating season coming up, I wanted to share a common issue and how to troubleshoot the Hall Effect switch. I’ve also included a chart with codes, what they mean, and what to look for when troubleshooting a furnace.
Tools needed:
Voltmeter capable of measuring DC volts
With the three-wire plug unplugged:
With power applied to the unit, unplug the three-wire plug and measure the pins as follows:
1. Connect the negative or black lead of the meter to pin three on the circuit board.
2. Connect the positive or red lead of the meter to pin one.
3. The meter should read between 7.5 and 10 volts DC.
4. Move the red or positive lead to pin two. It should read between 21 and 24 volts DC.
5. Replace the circuit board if these pins are outside these readings.
With the three-wire plug plugged in:
With power applied to the unit and the three-wire plug in place, measure the pins as follows:
1. Connect the negative or black lead of the voltmeter to pin three on the circuit board.
2. Connect the positive or red lead of the voltmeter to pin one.
3. Slowly rotate the induced draft motor. On half of one rotation of the induced draft motor shaft the voltage reading should be less than 1 volt DC. On the other half of one rotation the voltage reading should be between 6.5 and 9 volts DC.
4. If the voltage reading is not less than 1 volt DC on half of the rotation of the motor shaft and not between 6.5 volts and 9 volts DC of the other half of the shaft rotation, replace the Hall Effect Switch.
5. Move the red or positive lead to pin two. It should read between 16.5 and 21 volts. If not, replace the Hall Effect Switch.
Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.
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