Saunier Duval Sylva Fault codes & diagnostics

4 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 1997-2005 Discontinued

Replaced by: Thema Classic

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Just want the most common Saunier Duval Sylva faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 4 documented codes

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Air flow rate safety device

Your boiler has detected that air cannot flow correctly through the exhaust flue, so it has safely shut itself down to prevent any harmful gases from building up.

Emergency Engineer only £120-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi tube
Full explanation for Air flow rate safety device
Loss of water indicating light

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £0-£120

What to check first

  1. Locate the filling loop (usually a silver braided hose with one or two valves) underneath the boiler.
  2. Slowly open the valve(s) to allow mains water into the system while watching the pressure gauge.
  3. Close the valve(s) tightly once the needle reaches 1.5 bar.
  4. Restart your boiler.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for Loss of water indicating light
Overheating safety

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or safety issues.

High Engineer only £120-250

Parts commonly replaced

  • Limit thermostat
  • Pump
  • Diverter valve
  • Primary heat exchanger
Full explanation for Overheating safety
Safety lock-out

Your boiler has detected trapped air bubbles that are preventing it from running safely, so it has shut down to protect itself.

High DIY-safe £80-120

What to check first

  1. Check your radiator valves and make sure there are no leaks
  2. Use a radiator key to bleed any trapped air from your radiators until water flows out steadily
  3. Once bled, check your boiler pressure gauge and use the filling loop to top it back up to 1.5 bar
  4. Reset the boiler using the button on the control panel

Parts commonly replaced

  • Automatic air vent
  • System pressure gauge
  • Filling loop
Full explanation for Safety lock-out