How to Check a Heat Pump Reversing Valve Coil
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The person typing how to check a heat pump reversing valve coil with a multimeter wants to know whether the valve coil has failed, or whether the heat pump is not switching modes because the control voltage never reaches the coil. A multimeter can answer both questions in part. It can show whether the coil winding is open or shorted with the power off, and it can show whether the 24 V control supply is present at the thermostat and the control board. It cannot show whether the valve body is stuck. This guide covers the safety steps, the coil resistance check, the control voltage checks, and where a meter check ends.
Quick answer
- Work only with the power off at the main breaker and the disconnect, and confirm the power is off with the meter before you open any panel.
- Check the coil winding in resistance mode with the low-voltage leads removed. An open coil reads OL, and a shorted coil reads near 0 Ω.
- Check the 24 V control supply with AC voltage at the thermostat and control board while the system is powered and calling for a mode change.
- The sources opened for this guide do not give a coil resistance value or name the terminal that energizes the valve, so use the equipment’s wiring diagram and service data for both.
- A good coil reading does not prove the valve moves. A stuck valve can fail even when the coil is electrically sound.
Safety first
A heat pump is a mains-powered appliance, and its outdoor and indoor units contain high-voltage parts as well as the 24 V control circuit. Fluke’s general multimeter guide says to turn off circuit power and discharge any high-voltage capacitor before testing resistance, continuity, diodes or capacitance, and its 116 HVAC page marks the resistance and continuity steps as “power off first.” Treat every panel as live until your meter confirms that the power is off.
Some checks in this guide require the power on, with the panel open. Those readings are for a qualified HVAC technician. If you are not trained and licensed to work inside HVAC equipment, stop after the power-off checks and call a technician for the rest. Do not bridge the valve coil or the control terminals to force the valve to move.
What you need
- A digital multimeter with an ohms range, a continuity mode and an AC voltage range, rated for the voltages you will measure.
- The equipment’s wiring diagram, which shows the reversing valve coil and the control terminals on the board.
- A screwdriver set that fits the access panels and the valve coil connector.
- A photo or label for each low-voltage wire before you disconnect it.
- The model and serial number of the heat pump, so you can check the coil resistance and terminal assignments in the service data.
Step 1: Confirm the power is off and find the coil
Turn off the power at the breaker or disconnect that feeds the unit. Then use the multimeter to confirm the power is off, as Fluke’s guidance says, before you connect any leads. Do not rely on the thermostat setting alone. Once the panel is open, find the reversing valve coil. It is usually a solenoid coil on the valve body, with two low-voltage leads. The sources opened for this guide do not show the layout of any specific heat pump, so the model’s diagram decides which leads belong to the coil.
Step 2: Disconnect the coil leads and check the winding
With the power off, disconnect the two low-voltage leads from the coil, and keep them apart so that the reading is only for the coil. Set the meter to resistance (Ω). Touch one probe to each coil terminal. Fluke’s HVAC guidance for motor and winding checks uses the same logic: an open winding reads OL, and a shorted winding reads near 0 Ω. The 116 guidance describes open and shorted windings as faults that call for replacement, and the sources do not say the same for a valve coil, so confirm the action in the service data.
Fluke’s resistance guide notes that a solenoid’s resistance can change over time because of insulation breakdown and moisture. A coil that reads a little different from a new one does not automatically prove a fault. The sources do not give an expected resistance for a reversing valve coil, and any figure you find on a forum or a parts site is not one this guide can confirm.
Step 3: Check the 24 V control supply with the system powered
This step needs the power on. Fluke’s 116 HVAC guidance says to set the meter to AC voltage and measure at the thermostat terminals. The R to C reading should be about 24 V. When the thermostat calls for heating or cooling, the R to W or R to Y reading should also be about 24 V, according to the same guidance. Fluke’s guidance says the 24 V supply comes from the transformer, where the primary reads 120 V or 240 V depending on the system and the secondary reads about 24 V.
The sources do not say which board terminal energizes the reversing valve in a heat pump. Use the wiring diagram for your model to find that terminal, and measure it while the system is calling for the mode change. If the thermostat supply is present and the valve terminal has no voltage, the fault lies between them. If the voltage is present at the valve terminal and the coil reads open, the coil is the likely fault.
Step 4: Check for ghost voltage if the readings look wrong
Fluke’s 116 HVAC guidance describes ghost voltage, which can make a circuit look powered when it is not. The meter’s low-impedance AC voltage mode, LoZ, helps separate the two. The guidance says to compare the LoZ reading with the standard AC voltage reading, and that if the LoZ reading is significantly lower, the voltage is ghost voltage. Use the check when a control terminal reads close to 24 V but the valve does not respond. The sources do not give a rule for interpreting a LoZ reading in every heat pump, so treat the result as a prompt to check the circuit further, not a final answer.
Step 5: Read the results together
A coil that reads OL with the power off is open, and a coil that reads near zero is shorted. A coil that reads a value in the range shown in the service data, with 24 V present at its terminal when the system calls for a mode change, points away from the coil and toward the valve body or a mechanical fault. The sources do not say how to test a valve body, so the service data and a technician’s inspection are the next steps.
Write down each reading with its terminal names and the thermostat call that was active. A meter reading without the call that produced it is hard to compare later.
Common mistakes
- Checking the coil with the power still on or with the leads still connected to the board. Fluke’s guidance is to turn power off before resistance checks.
- Measuring the 24 V at the wrong terminal. The sources do not name the valve terminal, so check the wiring diagram first.
- Trusting a single resistance reading without the service data.
- Bridging the coil or the control terminals to force a mode change. This can damage the board and does not show whether the valve is stuck.
- Treating a good coil reading as proof that the valve moves. The sources note that mechanical faults can remain after the coil checks out.
How we chose
This guide uses three Fluke Learn pages that we opened on 2026-10-27. We chose the resistance steps because they apply to a coil winding, the HVAC voltage steps because they cover the 24 V control supply, and the safety steps from Fluke’s general guide. We did not find a Fluke or Klein page that covers reversing valve coils, so the method follows Fluke’s general resistance and HVAC steps, and the equipment’s own documentation decides the details.
What we could not verify
- The coil resistance for any reversing valve. Forum and parts-site figures were not confirmed against a manufacturer document, so this guide gives none.
- The board terminal that energizes the reversing valve in any particular heat pump.
- The steps to test the valve body for a stuck or leaking valve.
- Whether a coil check and a voltage check together rule out every control fault.
- The repair or replacement steps for a particular valve or board, which depend on the model.
FAQ
What does an OL reading on the reversing valve coil mean?
Fluke’s guidance says an open winding reads OL in resistance mode. Check that the leads are on both coil terminals and that the meter is in the right mode. If the reading stays OL with the leads disconnected from the board, the coil is open, and the service data decides the next step.
Can a good coil still leave the heat pump stuck in one mode?
Yes. The sources note that mechanical failure, such as grit or a worn diaphragm, can keep a valve from moving even when the coil is electrically sound. Check the coil, the control voltage and the valve body before you decide.
Why does the thermostat show 24 V but the valve does not switch?
The thermostat supply can be present while the board or the valve terminal has no output. Measure at the board terminal that feeds the valve during a mode call, and compare it with the wiring diagram.
Is it safe to take the readings with the panel open?
The power-off checks are safer than the powered checks. The powered voltage readings are made inside a mains-powered appliance, so they are for a qualified technician.
What is the next step after a bad coil reading?
Check the service data for the replacement coil part number and follow the manufacturer’s procedure. Do not fit a coil with a different voltage rating.
Bottom line
Cut the power at the main breaker, confirm it with the meter, and check the reversing valve coil winding for an open or a short. Then check the 24 V control supply while the system calls for a mode change. A coil reading does not rule out a stuck valve, and the sources give no resistance figure, so use the equipment’s service data for every number and follow a technician’s guidance on the powered checks.
Sources
- Fluke, “How to Measure Resistance with a Digital Multimeter”, https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance, opened 2026-10-27.
- Fluke, “Troubleshooting 10 Common Issues with the Fluke 116 HVAC Multimeter”, https://www.fluke.com/en-us/learn/blog/hvac/troubleshooting-10-common-issues-with-the-fluke-116-hvac-multimeter, opened 2026-10-27.
- Fluke, “How to Use a Multimeter: A Comprehensive Guide”, https://www.fluke.com/en-us/learn/blog/maintenance-monitoring/how-to-use-a-multimeter-guide, opened 2026-10-27.
