F6 error, indoor unit stops and may flash LEDs, no cooling or heating

Safety: Turn off power at the main breaker and outdoor isolator before touching any wiring.

• Verify wiring connections: Remove the indoor unit’s right‑side electrical cover and the outdoor unit’s terminal cover. Check that the communication terminals (often labeled S1/S2/S3 or L/N/S) match exactly between indoor and outdoor units.
• Tighten terminals: Gently tug each conductor. Tighten any loose screws. Ensure no copper strands are exposed or touching adjacent terminals.
• Check cable condition: Inspect the interconnecting cable along its route for cuts, rodent damage, or crushed sections. Replace the cable if any conductor is damaged.
• Polarity check: Confirm that the same color wire is used on the same terminal at both ends (e.g., brown to S1, blue to S2). Correct any mis‑wiring.
• Power cycle: Restore power, wait 5 minutes, then restart the system from the remote. Observe if F6 clears.
• If F6 remains: One of the PCBs may be defective. Further diagnosis (oscilloscope or protocol analysis) is required and should be performed by a Vaillant service technician.