Arkansas HVACR NewsMagazine January 2026

HVACR NewsMagazine January 2026

Tech News

Common Sensor Issues Coil temperature sensor failures are relatively uncommon. However, they're not impossible, and there may be other issues to look out for, including wiring problems. Thermostats in particular don't often fail on their own. They're not very prone to drifting, so most of the time, the issues are actually with loose or rubbed-out wires. A visual inspection will be able to tell you if there is a wiring issue. These can be resolved fairly easily by making sure the new wiring is tight, secured properly, and doesn't rub against anything. On the flipside, they are often not as accurate or precise as thermistors. Thermistors are a bit more prone to drifting than thermostats. When possible, the sensor itself should be on the underside of the coil so that the sun doesn't affect its readings. Radiant heat from the sun could cause the thermistor to pick up more heat than what's actually on the coil, thus inflating the resistance and preventing the thermistor from putting out a low enough resistance value for the board to recognize a need for defrost. In any case, you can still test the thermostats and thermistors to see if they're in good shape or need replacement. Some electronic boards, like the White-Rodgers Universal Defrost Control, may also display a fault code when they detect an issue with a sensor. Further diagnostics will be necessary to determine what is wrong with the sensor, but it gives you a solid start by identifying the coil sensor as the problem. Testing Coil Temperature Sensors You can test the coil temperature sensors with an ohmmeter or an electrical multimeter that can read in the kilohms scale. The methods for testing the two are slightly different.

Since the thermistor's resistance is constantly checked by the defrost board, the thermistor's resistance will also notify the defrost board when the coil is warm enough for the unit to come out of defrost within the set time limit. Thermistors come in different varieties based on their resistance. The most common type is a 10k (kilohm or kΩ) thermistor. The OEM may have a chart with the ohm readings and corresponding temperatures in their respective diagnostic or tech support app, but they're quite easy to find online as well. Many units that use thermistors as coil temperature sensors will actually have two: one that measures coil temperature and one that measures the ambient temperature. Systems that use demand defrost look for differentials between the two to determine when to put the unit into defrost. The White Rodgers Universal Defrost Control (47D01U 843) is one such example; its thermistors can also replace a thermostat.