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
Compressors are the heart of the refrigeration system. It creates the pressure difference to move the refrigerant through the system. Compressor checks are normally not done unless when troubleshooting the unit. Since compressors are expensive and their replacement can be time consuming, you need to be sure it is bad before replacing it. There are certain ways to identify compressor problems and measure the compressor windings and identify unmarked terminals.
One example would be a seized compressor. The compressor draws locked rotor amps and hums for several seconds and the internal overload opens up. This can be caused by a mechanical or electrical problem within the compressor or by an external electrical problem. The unequaled refrigerant pressures can cause a compressor to pull lock rotor amps and appear to be seized.
Also, low incoming supply voltage to the compressor which can be caused from poor contacts on a contactor. The run or start capacitor to the compressor could be open or weak allowing the appearance of a seized-up compressor.
The scenarios above give the appearance of a seized compressor but are not and are repairable without replacing the compressor. If the mechanical workings of the compressor fail or brake causing a sized up compressor the compressor will have to be replaced.
Another example would be low or no capacity. Compressors have valves, rings and plates that seal between the high – pressure and low – pressure sides of the system and the compressor. If either is damaged, correct operating pressures can never be developed. This condition can be checked two ways, one using pressure gauges and an amp meter. Using the pressure gauges, you would notice that the suction pressure is excessively high and the discharge pressure is excessively low, the compressor most likely has an internal problem that allows pressure to leak past the high and low sides of the compressor. Using the amp meter, you would notice a considerably lower than normal amp draws. The compressor would have to be replaced.
Checking Compressor Windings
If you have ever had a single-phase compressor that the terminals were not marked there is a fairly simple way to too identify which terminal is Common (C), Start (S), and Run (R). First follow all safety precautions, make sure power is off and if the compressor is in a pressurized system, do not place the meter probes directly on the compressor terminals. If the terminals are damaged and the system is pressurized, disturbing them to take readings could cause them to blow out, causing injury. Second remove the three compressor wires from all components. Then using your meter measure across the three compressor leads to find the two terminals that have the highest resistance reading. The remaining terminal is the Common (C) winding terminal. Next put one of your meter leads on common that was just found and the other lead on one of the other two remaining terminals to find the next highest resistance measurement. This is the Start (S) winding terminal. The last remaining terminal is the Run (R) winding terminal.
Compressor testing is needed to determine what is wrong and what may have caused the problem. There are some compressor symptoms that appear to be a compressor failure but are caused by another part on the system. Above is a brief example of compressor problems and how to check windings on a good compressor. There are a lot more system related problems that, if not corrected, can cause mechanical and electrical failure of a compressor.
Sources:
HVAC Servicing Procedures
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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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