F03 error and softener stuck in regeneration or cannot complete cycle

Safety: Disconnect the softener from mains power before removing the control head cover.

Diagnostic steps:

• Listen for motor operation: When a regeneration is started, listen for the drive motor. A humming sound without movement suggests a jammed valve or failing motor.
• Inspect valve rotor: Remove the control head cover. Observe the rotor/valve disc while initiating a manual regeneration. If it does not turn or stops abruptly, check for mechanical obstructions or foreign objects in the valve body.
• Check motor gears: Many Vaillant heads use a small gearmotor. Inspect the plastic gears for cracks or missing teeth. Replace the motor/gearbox assembly if damaged.
• Verify position sensor: Some models use a microswitch or optical encoder to detect valve position. Ensure the wiring harness is firmly connected and that the sensor is correctly aligned with the cam or rotor. Replace the sensor if it fails continuity tests.
• Lubrication and reassembly: Lightly lubricate O‑rings with silicone grease only (never petroleum‑based products). Reassemble the head, restore power, clear the F03 code, and run a full regeneration to confirm proper operation.

Warning: Forcing the rotor by hand can damage the sealing surfaces. If the valve is seized, replacement of the complete control head may be required.