Smart thermostat shows F.08 and hot water cylinder temperature is not displayed; hot water control limited.

Safety: Isolate power to the boiler and controller. Cylinder surfaces may be hot; wear appropriate protection.

• 1. Locate cylinder sensor: Find the Vaillant cylinder sensor inserted into the dry pocket on the hot water cylinder. Ensure it is fully inserted and secured.
• 2. Inspect cable: Follow the sensor cable back to the controller. Look for cuts, crushed insulation, or loose connections at the sensor terminals.
• 3. Measure resistance: With power off, disconnect the sensor and measure its resistance. Compare to Vaillant’s NTC chart for the approximate cylinder temperature. Replace the sensor if readings are out of range.
• 4. Check for shorts/opens: If resistance is infinite or near zero, the sensor or cable is faulty. Replace the sensor and, if necessary, the cable run.
• 5. Reconnect and test: Reconnect the new or tested sensor, restore power, and verify that the thermostat now shows a realistic cylinder temperature and F.08 has cleared.