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 technology advancing more and more every day, the days of relays to contactor style control are behind us. Carrier units with MBB boards have sensors everywhere from the suction pressor to Current sensors on the compressor. This does have a great benefit to having a unit in which you can attach one set of remote contacts then it will run. But this can cause issues once you start getting wear on the sensors. This can cause the codes P.16-P.27 which can cause massive headaches to hunt down.
The Code states the Compressor has not started or pressure has not been established. This Fault is read if the compressor discharge-suction did not increase 10 psig in 2 min. This goes back to the sequence of operation for the starting of the compressor. The sensor that makes a technician’s job easier on checking trails is also how the machine knows it’s doing what the MBB commands. This can affect the unit not being able to run at full capacity to shutting down whole circuits. This can come from faulty transducers to faulty current sensors. This will make the MBB Board think the compressor is not operating correctly
The potential fixes for this are very simple if you take it step by step. The first step is to verify the standing pressure. Use a set of gauges tied in the service ports next to the transducer. Once the gauges are installed use the Comfort Link to verify the pressures of the Gauges. If they are off more than 10 psi the bad transducer needs to be changed. If the transducer’s reading is good verify the control voltage to the contactors and make sure there is no loss going to the contactors from the MBB. Make sure you have the signal out of the scroll protection module. If those are present and the transducer is fixed the problem is solved.
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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