Reversing Valve Troubleshooting — Heat Pump Stuck in Heat or Cool Mode

Common Symptoms

• Heat pump blowing hot air in cooling mode
• AC running but apartment or house is getting hotter
• Thermostat set to COOL but supply air is warm or hot
• Cooling mode but heating — system stuck in heat mode
• Heat pump blowing cold air in heating mode
• O wire disconnected — reversing valve not energizing
• Valve stuck in heat mode despite thermostat calling for cool
• Outdoor unit runs but house won't cool down
• Emergency heat works fine but normal heat pump operation doesn't
• System cycles normally but supply air temperature is wrong for the mode

Most Likely Causes

1. 1

   Loose or Disconnected O/B Connector (Most Common)

   The single most common field cause. The O terminal (orange wire) at the outdoor unit control board connects via a push-on spade connector. Vibration, moisture, and repeated thermal cycling cause it to work loose over time. When the connector backs off even slightly, the 24V signal is lost, the reversing valve solenoid de-energizes, and the valve stays in its spring-default position — heating on most brands. Inspect this connector first on any heat pump stuck in one mode.

2. 2

   Failed Reversing Valve Solenoid

   The solenoid coil that shifts the 4-way valve can fail open-circuit (no resistance, no magnetic field) or shorted. Measure across the solenoid terminals with a multimeter in resistance mode — a good solenoid reads 10–100 ohms depending on brand. An open-circuit solenoid (OL on the meter) means the coil is burned out. The solenoid can be replaced independently of the valve body on most units.

3. 3

   Mechanically Stuck Reversing Valve Spool

   The internal spool that redirects refrigerant flow can stick in one position due to oil contamination, refrigerant flash, or corrosion. Even with 24V correctly applied to the solenoid, the valve doesn't shift. Diagnosis: confirm 24V at the solenoid terminals, confirm the solenoid coil has correct resistance, and listen for a click/thump when power is applied. No click with correct voltage = stuck spool. Remedy is usually valve replacement.

4. 4

   Wrong O/B Jumper Setting on Thermostat

   Smart thermostats (Ecobee, Nest, Honeywell T6 Pro) have an O/B configuration setting that determines whether the reversing valve wire is O (energize to cool) or B (energize to heat). If set wrong after a thermostat replacement, the system operates in reverse — heating when cooling is called, and cooling when heat is called. Verify the O/B setting matches your heat pump brand's wiring convention.

5. 5

   Low Refrigerant / Refrigerant Restriction

   Severe refrigerant loss can prevent the reversing valve from shifting due to insufficient pressure differential across the valve. The valve requires a minimum pressure difference between high and low side to shift reliably. This is less common but should be considered if all wiring checks out and the solenoid is confirmed good.

Quick DIY Checks

1. 1Set the thermostat to COOL and let the system run for 5 minutes. Go to a supply register and hold your hand in the airflow. If the air is warm while the outdoor unit is running in cooling season, the reversing valve is not shifting to cooling mode. This confirms the diagnosis.
2. 2Turn off the thermostat and the outdoor unit disconnect. Open the outdoor unit control box (4–6 screws on the access panel). Locate the O terminal on the control board — it should have an orange wire connected. Inspect the spade connector: look for it to be pushed back, corroded, or hanging loose. Reconnect firmly and restart the system. This fixes the problem in many field calls.
3. 3With the system calling for cooling and running, set your multimeter to AC voltage (200V range). With power on and thermostat calling for cool, test between the O terminal and C (common) terminal on the outdoor control board. You should read 24V AC. No voltage = thermostat, wiring, or control board issue. 24V present but valve not shifting = solenoid or stuck valve.

1. 4Locate the reversing valve solenoid — it's a small cylindrical coil clipped onto the 4-way valve body on the refrigerant lines. Turn off power. Disconnect the two wires from the solenoid. Set a multimeter to resistance (ohms) mode and measure across the solenoid terminals. Good range: 10–100 ohms (check your service manual for exact spec). OL (open loop) = burned-out solenoid, needs replacement.
2. 5To test if the valve spool is mechanically stuck: with power restored and 24V confirmed at the solenoid, listen and feel the reversing valve body for a click or vibration when the thermostat switches modes. A functioning valve clicks audibly. No click with correct voltage and correct solenoid resistance = stuck spool. The valve body needs replacement — this is a refrigeration repair requiring EPA 608 certification.

Repair vs Replace

Recommended Tools & Parts

• Reversing Valve Solenoid Coil

  Replacement solenoid coil for 4-way reversing valves. Clips onto the valve body without opening refrigerant lines. Match the coil voltage (24V) and connector type to your valve. Common brands: Ranco, Sporlan, Sanhua.

  $20–$60

  Buy on Amazon →

• 4-Way Reversing Valve (Ranco / Sporlan)

  Complete reversing valve body for heat pump refrigerant circuit. Requires refrigerant recovery, brazing, and recharge by an EPA 608 certified technician. Match the valve model to your unit's service manual.

  $80–$200

  Buy on Amazon →

• Digital Multimeter

  Required for testing 24V AC at the O terminal, solenoid resistance, and confirming thermostat output. Use AC voltage mode for 24V control circuit checks.

  $20–$50

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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