HVAC Technician Academy
Learn how to diagnose O/B control problems, failed reversing-valve coils, stuck valves, partial shifting, and mode-change failures without guessing.
First verify whether the thermostat and control board are sending the correct O/B signal. Then verify proper voltage at the reversing-valve coil, check coil resistance with power removed, and confirm whether system pressures and line temperatures actually change when the valve is commanded. A control failure, coil failure, and mechanically stuck valve can produce similar symptoms.
A heat pump that stays in the wrong mode does not automatically have a bad reversing valve.
The failure can be:
The reversing-valve control is commonly connected to an O/B thermostat terminal.
Different equipment manufacturers may energize the reversing valve in different operating modes.
Therefore:
Study: Heat Pump O/B Thermostat Wiring Guide
The solenoid coil creates a magnetic field when energized.
That magnetic action pilots the reversing valve so refrigerant flow changes direction.
A failed coil can prevent shifting even when the mechanical valve body is still functional.
If the valve should be energized in the current mode, measure directly across the coil.
Possible patterns:
| Reading | Possible Direction |
|---|---|
| Correct control voltage, no shift | Coil or mechanical valve problem |
| No control voltage | Thermostat, wiring, board, or configuration issue |
| Low/unstable voltage | Control-circuit voltage drop or transformer/load issue |
With power removed and the coil isolated, check coil resistance.
An open reading may indicate a failed coil.
Exact expected resistance varies by design, so manufacturer data should be used when available.
Electrical voltage at the coil does not prove the valve actually shifted.
After a valid mode-change command, verify the refrigeration circuit response.
Depending on the equipment and operating condition, evaluate:
If the coil is correctly energized but the system does not change modes, a mechanical valve problem becomes more likely.
Possible symptoms include:
A reversing valve does not always fail completely.
Internal leakage or incomplete movement can produce poor capacity even though the system appears to change modes.
Symptoms can include:
Diagnose the full refrigeration circuit before concluding the valve is bypassing internally.
Technician Case File
Thermostat: set to cooling.
Compressor: running.
Indoor air: warm.
Technician initially suspects a stuck reversing valve.
O/B voltage at thermostat: present.
O/B voltage at outdoor unit: missing.
Field wiring is found open between indoor and outdoor equipment.
Lesson: The valve was never receiving the command to shift.
Correct control voltage reaches the valve coil.
Valve does not shift.
Power is removed and coil is isolated.
Coil tests open.
The control circuit is functioning. The solenoid coil itself has failed.
Correct O/B command: verified.
Coil voltage: correct.
Coil resistance: reasonable.
Mode command changes.
System pressures and line temperatures remain essentially unchanged.
After the electrical side is proven, suspicion moves toward mechanical valve operation or another refrigeration-side problem.
1. Verify the required operating mode.
2. Verify manufacturer O/B energization logic.
3. Confirm thermostat configuration.
4. Measure O/B output at the thermostat or control board.
5. Measure control voltage at the outdoor unit.
6. Measure directly across the reversing-valve coil.
7. If needed, de-energize and test coil resistance.
8. Command a mode change.
9. Verify pressure and temperature response.
10. Condemn the mechanical valve only after the electrical command and coil operation have been proven.
| Pattern | Possible Direction | Next Test |
|---|---|---|
| No O/B signal | Thermostat/configuration/control issue | Trace O/B circuit |
| O/B at thermostat, none outside | Field wiring/control path | Trace wiring and connectors |
| Correct voltage + open coil | Failed solenoid coil | Verify specification and repair |
| Correct voltage + good coil + no refrigerant change | Possible mechanical valve problem | Evaluate pressures and temperatures |
| Valve shifts but capacity poor | Possible partial shift, bypass, or unrelated refrigeration fault | Complete system diagnosis |
A reversing valve diagnosis requires two proofs: prove the electrical command reached the valve, then prove the refrigeration circuit responded.
1. Should you assume every heat pump energizes O/B in the same mode?
2. Where should control voltage be measured when testing the coil?
3. What does correct coil voltage with no system mode change suggest?
4. Can a reversing valve partially shift?
5. What must be proven before condemning the mechanical valve?
1. No. Verify the specific manufacturer's energization logic and thermostat configuration.
2. Directly across the reversing-valve coil terminals.
3. The coil or mechanical valve becomes more suspicious after the control path is proven.
4. Yes. Incomplete shifting or internal leakage can create poor performance.
5. Correct control signal, proper coil operation, and lack of expected refrigeration response.
Return to the heat-pump training hub.
O/B Wiring Guide →Review thermostat and reversing-valve control wiring.
24-Volt Controls →Trace low-voltage control signals under load.
Heat-pump diagnosis may involve live electrical testing, high refrigerant pressure, hot discharge lines, rotating fans, and energized controls. Use properly rated instruments, appropriate PPE, and manufacturer procedures. De-energize equipment before resistance testing or wiring changes.
Thermostat configuration, O/B wiring, low voltage, coil failure, and mechanical valve faults can all create similar symptoms. Prove the command before condemning the valve.
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