Display shows Err3 and unit will not complete regeneration; valve appears stuck in one position

Safety first: Unplug the softener and place it in bypass. Relieve pressure by opening a faucet before removing any valve components.

Step-by-step checks:

• 1. Inspect for recent work: If the valve was recently serviced, verify that the cam and rotor were reassembled according to the Marlo service manual. Misalignment can trigger Err3.
• 2. Remove rotor/seal pack: Remove the screws holding the valve cover and carefully pull out the rotor and seal pack assembly. Note the orientation for reassembly. Inspect the rotor face for scoring, grooves, or warping.
• 3. Check seals and spacers: Examine the seal pack and spacers for swelling, tearing, or flattening. Damaged seals can cause excessive friction and prevent proper indexing.
• 4. Clean valve body: Inspect the valve body ports for resin beads, sand, or debris. Flush with clean water and wipe with a lint-free cloth. Do not scratch the sealing surfaces.
• 5. Lubricate and reassemble: Apply a thin film of NSF-approved silicone grease to the rotor O-rings and seals. Reinstall the rotor/seal pack and ensure the cam is aligned to the correct home position per Marlo instructions.
• 6. Test operation: Restore power and take the unit out of bypass. Initiate a manual regeneration and observe whether the valve now indexes through all cycles without error.

When to replace parts: If the rotor is visibly worn or the seals are deformed, replace the complete Marlo rotor and seal pack kit. Persistent Err3 after mechanical service may indicate a control board indexing logic fault, which should be evaluated by a technician.