Overview
S.42 on a Vaillant EcoTEC Pro / EcoTEC Plus means the boiler has detected a problem with the flue non‑return flap (also called a flue back‑draught or anti‑recirculation flap) or its return signal and has blocked burner operation. The flap’s job is to stop combustion products and outside air from flowing back into the appliance; if the flap is stuck, wrongly positioned, blocked or the flap mechanism/sensor isn’t reporting correctly to the PCB, the boiler will lock out for safety. The code is a status/safety code rather than a simple comfort fault, because any issue with the flue risks combustion products — including carbon monoxide — not leaving the building correctly. Causes can be mechanical (flap jammed by debris, bird nests, ice, soot or corrosion), electrical (motor/actuator failure, wiring or connector fault), or electronic (faulty position sensor or PCB interpreting the flap signal incorrectly). It can also be caused temporarily by external conditions such as strong wind causing flue gas recirculation. Severity is moderate to high — the boiler has shut down the burner for safety, and you should treat it seriously. Homeowners can carry out basic external checks and a single reset, but diagnostic testing, repairs or any work involving gas, the flue internals, wiring or the PCB must be carried out by a qualified Gas Safe engineer.
Possible Cause: Flue non return flap return signal blocks burner operation
Troubleshooting Steps
Safety first
- If you or anyone in the property smells gas or has symptoms of carbon monoxide (headache, nausea, dizziness, flu‑like symptoms), turn everything off, ventilate the building, leave immediately and call your gas emergency line and/or emergency services. Do not use the boiler or any gas appliance until safe.
- Make sure you have a working carbon monoxide alarm fitted near the boiler and sleeping areas.
- Before doing any checks, switch the boiler to OFF at the programmer and isolate electrical supply at the external isolator if accessible.
Initial homeowner checks (safe, non‑invasive)
1. Note the exact code and whether the boiler allows a single automatic reset or is locked out. Try one reset only: switch the boiler off, wait 30 seconds, switch back on and attempt a restart following the manufacturer’s reset procedure. If the code returns, do not keep resetting repeatedly.
2. Visually inspect the external flue terminal (outside the property). Look for obvious blockages: leaves, nesting material, bird boxes, heavy soot, or ice. Remove only external debris you can safely reach from ground level; do not climb or insert tools into the flue.
3. Check for signs of wind or back‑draught: after a storm or in very windy conditions, the flap can stick or flue gas can be blown back. Note if the fault is intermittent and coincides with windy weather.
4. Listen at the boiler on restart for the fan running and any unusual rattling or clicking that could indicate a stuck flap or failed actuator.
5. Check the boiler display for any other accompanying codes (fan fault, ignition errors) and check mains power and the boiler isolator/fuse to ensure the unit is properly powered.
Further diagnostic guidance (do not open or attempt internal repairs unless qualified)
1. If the external terminal is blocked and you have cleared it safely, try restarting once. If the code clears and stays off, arrange a service to inspect the flap mechanism and confirm no damage.
2. Do not attempt to force or lubricate the flap from inside the flue; the flap is part of the sealed combustion path and must not be altered by unqualified persons.
3. Any inspection involving the flue internals, flap actuator, wiring, connectors or PCB should only be done by a Gas Safe registered engineer. They will check flap motor operation, position sensor signal, wiring continuity, the fan operation and for signs of recirculation or flue blockage. They can test the flap with the boiler powered under controlled conditions and measure the return signal to the PCB.
4. If the engineer finds the flap mechanism is jammed (debris, corrosion, broken hinge) the flap assembly or actuator will likely require replacement. If the flap and actuator test OK, the engineer will check the sensor, wiring harness and control electronics for a fault.
What to tell the engineer / prepare before the visit
- Give the exact fault code S.42 and any other codes shown, describe whether the fault is constant or intermittent and when it first appeared.
- Tell them what you inspected yourself (external terminal clear/blocked, recent windy weather, one attempted reset) and whether the boiler made any unusual noises.
- Have your boiler model, serial number and last service date ready.
Final advice
- Do not use the boiler until a qualified engineer has confirmed the appliance is safe. Repeatedly attempting to run the boiler while a flue fault exists risks unsafe flue operation.
- Arrange an appointment with a Gas Safe registered engineer or Vaillant‑approved service technician to diagnose and repair the fault. Only a qualified engineer should carry out electrical, flue or gas‑related repairs.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.