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
HVAC technicians often face challenges in pinpointing intermittent or elusive issues, as they can be harder to diagnose compared to consistent problems. Identifying and resolving issues related to intermittent malfunctions or sporadic performance can be more time consuming for technicians. I want to go over some of the most common intermittent malfunctions for gas furnaces that I have come across this winter, and I want to share and elusive issue I came across this past summer.
A common cause for a furnace to experience a high limit fault is restricted airflow. This can result from issues such as a dirty or clogged air filter, blocked vents, or obstructed ductwork. When air flow is restricted, heat builds up inside the furnace, triggering the high limit switch to shut off the burners to prevent overheating. Regular maintenance, including replacing air filters and ensuring proper ventilation. If the system has zoning, check to make sure all of the dampers are working properly. Take voltage readings at the zone board. I’ve see a zone board short cycle the blower motor in first and second stages. Test voltage and amp readings at the blower motor. If the system has a VFD drive, go through the parameters and make sure everything is set to the specifications to that particular motor.
A pressure switch fault is often linked to issues with the ventilation system. The pressure switch monitors the airflow to ensure proper ventilation during the combustion process. If there is a blockage or restriction in the venting system, the pressure switch may not detect the expected airflow. Common causes of a pressure switch fault include:
To check the inducer fan motor in a furnace, You can follow these steps:
I’ve seen the gas valve cause intermittent issues. Use a manometer to check the inlet gas pressure and outlet pressure. Take pressure readings during first stage and during second stage. You can also take an Ohm reading across the terminals. Most gas valves have an Ohm rage between 20 – 40 Ohms.
I want share one elusive issue I came across this past summer. A homeowner had installed an air conditioning system to his fully finished garage a few years ago. The garage was professional finished with insulation and dry wall. The system worked great with no problems for years. This past summer was the first time they had any issues with it. It was now running constantly and there was sweat forming on the supply vents. I thought it could be and airflow issue or a refrigerant issue. We first checked to make sure all the ductwork was clear and free flowing, the filter was brand new, and the coil was clean. We then checked pressures, superheat, and subcooled temperatures. Everything thing was in range. We then asked the homeowner if there were any changes made to the house or the garage. It ended up that the homeowner had a large oak tree cut down and that was enough to change the load on the house.
Sometimes us HVAC technicians can’t be completely focused on just the system. When we get stumped on an issue, we need to take everything into account.
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.
Most gas furnaces have several safety limits, main limits, roll-outs, 80%’s will have a DSS (Daft Safety Switch) These safety
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