Error S.39

Safety precautions:

1) If you smell gas or suspect a gas leak, leave the building immediately and call your gas emergency number. Do not operate electrical switches or the boiler.

2) Isolate electricity to the boiler before removing covers for any inside inspection (turn off at the fused spur). However, avoid opening sealed sections or working on gas components unless you are a qualified engineer—this can void warranty and is dangerous.

3) Wear gloves and eye protection if you handle condensate or remove panels. Keep children/pets away while you work.

Initial homeowner checks (safe, simple steps):

1) Note the exact display and any other symbols. Try a normal reset: press the boiler reset button once or switch the boiler off at the spur for 30 seconds, then back on. If the code returns immediately or after a short cycle, further checks are needed.

2) Check the condensate discharge route: follow the small plastic condensate pipe from the boiler to its drain or outside outlet. In cold weather the external pipe can freeze—inspect for ice and, if frozen, carefully thaw with warm (not boiling) water or a warm cloth.

3) Locate the condensate trap/pump (usually a small clear bottle/container or a small pump unit near the base of the boiler or externally). Listen for the pump running when the boiler tries to fire. If the pump is full of water or the float is stuck, it can trigger the burner-off input.

4) Check for obvious loose wires or disconnected plugs at accessible external components (condensate pump plug, external interlocks). Do not probe wiring inside the sealed boiler casing unless qualified.

Specific diagnostic and basic fix steps you can try:

1) Reset and observe: Reset the boiler and immediately watch/listen. If the boiler attempts ignition and the code reappears when the condensate pump should operate, suspect the condensate pump/float or its wiring.

2) Condensate pump/trap cleaning: With the boiler powered off at the isolating switch, remove and inspect the condensate trap or small pump container. Empty any water, remove debris or slime, and check the float moves freely. Reassemble and power up, then reset the boiler. If the pump had been full/blocked this may clear the S.39 condition.

3) Thaw/clear condensate pipe: If the condensate pipe outside is frozen or blocked, clear and restore drainage. After clearing, run a reset and check if the code clears.

4) Check pump power/operation: If safe to do so and the pump is external, restore power and ask for heat (or a reset) while you watch the pump. Does it run and discharge water? If it does not run but has power, or the float does not operate, the pump is likely faulty.

5) External interlocks/room stats: Confirm any room thermostat, cylinder stat, frost stat or external timer is calling for heat when you expect it. If an external device is wired into the burner-off contact, it could be stopping ignition.

6) If the S.39 is caused by an over-temperature or safety thermostat: check for signs of overheating (very hot flow pipe, recent loss of circulation). If the boiler has recently lost pump operation or the system has been starved of water/flow, a safety thermostat may have opened. These checks usually require a qualified engineer.

When to call a professional and what to tell them:

1) Call a Gas Safe registered heating engineer or your installer/Vaillant service if a reset does not clear the code, if the condensate pump is blocked/damaged, if the pump will not run, if wiring/interlocks appear faulty, or if you are not comfortable performing the basic checks above.

2) If the boiler is under warranty, contact the installer or Vaillant-authorised service first—explain the S.39 status, what initial checks you performed (reset, condensate check, thawing), and whether the pump ran or was blocked.

3) Avoid attempting internal repairs, gas work, or replacing safety thermostats yourself. These must be performed by a qualified engineer to preserve warranty and safety.

Final note: S.39 is a protective shutdown (burner disabled) and can sometimes be cleared by simple condensate or reset actions, but persistent or repeat occurrences indicate a faulty condensate pump, stuck float, wiring/connector issue, or a triggered thermostat that needs professional diagnosis and repair. Contact a qualified engineer if the steps above do not clear the condition or if you are unsure.