Indoor unit runs but no heating in heat pump mode, blows cool or lukewarm air

Safety: Isolate power before accessing the outdoor unit. The reversing valve is part of the sealed refrigerant circuit and must not be opened by unqualified persons.

• Check mode change: Switch between COOL and HEAT modes and listen at the outdoor unit for a distinct change in refrigerant flow noise. Lack of change suggests the valve is not shifting.
• Inspect coil wiring: With power off, locate the reversing valve solenoid coil on the outdoor unit and check its connector and wiring for damage. Reseat the connector.
• Measure coil resistance: Using a multimeter, measure the coil resistance. An open circuit indicates a failed coil that must be replaced.
• Tap test (gentle): With the unit off, gently tap the valve body with a rubber mallet to free a possibly stuck slide. Do not use excessive force.
• Professional service: If the valve remains stuck or the coil is defective, a Vaillant technician must replace the reversing valve assembly and recharge the system.