F.16
Vaillant
Control Signal Output Fault
Smart thermostat shows F.16 and the boiler does not respond to temperature changes or schedules.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Failed output transistor/relay on thermostat PCB, Internal software lockup, Overload from incorrect load connected to control output, Damage from transient overvoltage
How to Fix / Troubleshooting
Safety: Isolate mains power to the boiler and thermostat before any wiring checks.
- 1. Power cycle: Turn off power to the system for 2–3 minutes, then restore and test if control resumes.
- 2. Check load on control output: Ensure the thermostat output is only connected to the intended Vaillant boiler control terminals or low‑current relay input, not directly to pumps or valves.
- 3. Inspect wiring for shorts: Look for crushed cables or moisture in junction boxes that could have shorted the control output.
- 4. Test boiler independently: Temporarily bridge the boiler’s room stat terminals (if safe and per manual) to confirm the boiler itself can fire. If it does, the thermostat output is likely faulty.
- 5. Replace thermostat: If F.16 persists and the boiler responds to a manual bridge, replace the Vaillant thermostat.
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Repair Difficulty
Medium
3/5
Required Part
Vaillant smart thermostat main PCB
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