Testing a heat pump’s reversing valve involves three main checks: verifying the solenoid coil gets power and has proper resistance, measuring temperature differences across the valve’s ports, and observing refrigerant pressure behavior. These tests help you determine whether the valve itself is failing, the coil is bad, or the problem lies elsewhere in the system.
Know the Symptoms First
Before pulling out your meter, it helps to confirm the reversing valve is actually your suspect. The most common sign of a failing reversing valve is a heat pump stuck in one mode. It heats fine but won’t cool, or vice versa. You might also notice the system running but delivering weak output in one mode while performing normally in the other. Strange noises during the mode switch, particularly a sustained hissing or whooshing sound, can indicate the internal slide isn’t seating properly and refrigerant is bypassing through the valve.
These symptoms overlap with other problems (a bad thermostat wire, a failed defrost board, even a weak compressor), so testing the valve directly is the only way to be sure.
Test the Solenoid Coil
The reversing valve is controlled by a small electromagnetic coil that slides an internal piston when energized. If this coil fails, the valve can’t shift, and the system gets stuck in whichever mode the valve defaults to. This is the easiest and most common test to start with.
Check for 24 Volts at the Coil
With the system running and the thermostat calling for the mode that should energize the valve, use a multimeter set to AC voltage and probe the two wires feeding the solenoid coil. You should read approximately 24 volts. Most residential heat pumps use a 230-volt to 24-volt transformer to power the control circuit, and that 24 volts is what energizes the reversing valve solenoid.
One important detail: most manufacturers energize the solenoid in cooling mode, so the coil should be powered when the thermostat calls for cooling. Rheem and Ruud systems are the main exception. They energize the solenoid in heating mode instead. If you’re not sure which applies, check the wiring diagram on the unit’s access panel. Getting this wrong will lead you to think the coil isn’t getting voltage when it simply isn’t supposed to be powered in that mode.
If you read 24 volts at the coil and the valve still isn’t shifting, the coil or the valve’s mechanical internals are the problem. If you read zero volts, the issue is upstream: a broken wire, a bad connection at the circuit board, or a thermostat that isn’t sending the signal.
Check Coil Resistance
To test the coil itself, turn off all power and disconnect the coil from the wiring. Set your multimeter to ohms and measure across the coil’s two terminals. A healthy 24-volt solenoid coil typically reads in the range of several hundred ohms. A reading around 519 ohms, for example, is perfectly normal. What you’re looking for are the extremes: an open circuit (infinite resistance or “OL” on your meter) means the coil’s winding is broken, and a reading near zero ohms means it’s shorted. Either result means the coil needs to be replaced. The good news is that solenoid coils can be swapped without recovering refrigerant, since they slide off the valve body externally.
The Temperature Differential Test
This is the key mechanical test for determining whether the valve is leaking internally. Even when the slide moves to the correct position, the valve’s internal seals can wear out over time, allowing hot high-pressure refrigerant to bleed into the low-pressure suction side. The system will run but with noticeably reduced capacity.
To perform this test, you need two pipe clamp thermometers or a dual-probe thermometer. In heating mode, measure the temperature of the suction line coming from the outdoor coil and compare it to the temperature at the suction port on the bottom of the valve (the line going back to the compressor). If the difference between these two readings is more than 3°F, the valve is bypassing internally and needs to be replaced.
In cooling mode, run the same comparison on the reversing valve’s ports. Again, a temperature difference greater than 3°F between the two suction-side connections indicates internal leakage. This 3°F threshold is a widely used benchmark in the field. Anything under that is normal heat exchange from the valve body itself.
Make sure the system has been running for at least 10 to 15 minutes before taking these readings. You want the system at steady state so the temperatures are stable and representative.
Pressure and Amperage Checks
Refrigerant pressure behavior can reveal a bypassing reversing valve, but it also helps you rule out a bad compressor, since the two failures look similar at first glance.
When a reversing valve is bypassing internally, high-pressure refrigerant leaks back to the low side through the valve. This reduces the pressure differential between the high and low sides of the system. Because the compressor’s workload depends on that differential, the compressor’s amp draw will be abnormally low. So if you measure amps at the compressor and the draw is well below the rated load amps on the data plate, either the compressor is failing or the reversing valve is bypassing.
To distinguish between the two, valve off (close the service valve on) the high side of the system and watch the low-side pressure. If the low side drops and the compressor pulls the refrigerant back as expected, the compressor is working and the reversing valve is your culprit. If the low-side pressure doesn’t drop when the high side is valved off, the compressor itself has failed internally. This is one of the most reliable ways to separate a valve problem from a compressor problem when the symptoms overlap.
The Magnet Test for Slide Movement
If you want a quick, non-invasive way to confirm whether the internal slide is actually moving when the solenoid energizes, a small rare-earth magnet works surprisingly well. Place a quarter-inch neodymium magnet on the outside of the valve body. The magnet will be attracted to the steel slide inside the valve and will physically move along the body when the slide shifts position.
Energize and de-energize the solenoid while watching the magnet. If it moves from one side to the other, the slide is traveling. The total distance of the slide’s throw varies by valve size and manufacturer, but you’ll see a clear lateral movement if the internals are working. If the magnet doesn’t budge, the slide is stuck, which could mean the solenoid isn’t generating enough force, there’s debris or sludge jamming the slide, or the differential pressure across the valve is too high for the pilot to shift it.
Putting the Tests Together
A logical sequence saves time. Start with the solenoid coil since it’s the most accessible and most commonly failed component. If the coil checks out electrically (proper resistance and receiving 24 volts), move to the temperature differential test to check for internal bypassing. If temperature readings are within 3°F but the system still isn’t switching modes, use the magnet test to see if the slide is physically moving. And if you’re seeing weak performance in both modes with low compressor amp draw, use the pressure isolation test to determine whether you’re dealing with a valve bypass or a compressor issue.
Replacing a reversing valve requires recovering refrigerant and brazing, so confirming the diagnosis before committing to the repair is worth the extra few minutes of testing. A bad solenoid coil, on the other hand, is a 15-minute fix with no refrigerant work involved, which is why it’s always worth checking first.