Overview
Error 105 on an ATAG boiler means the boiler has started its venting program because power was switched on or the power was interrupted. The venting program is a timed safety/purge routine (approximately 7 minutes) that runs the fan and exhaust sequence to clear the combustion chamber and flue of any residual gases or unburnt fuel before normal ignition is allowed. If the code appears only at power-up and the boiler completes the 7-minute sequence and then returns to normal operation, this is usually a normal and low-severity behaviour. If the venting program runs continuously, fails to finish, repeats frequently after short power blips, or is accompanied by other fault codes (fan faults, ignition faults, pressure errors), it indicates an underlying problem that needs investigation. Basic checks can be done by a homeowner, but because this concerns combustion and flue safety, any remedial work beyond simple checks or thawing a condensate pipe should be carried out by a qualified Gas Safe engineer or ATAG-approved technician.
Possible Cause: Venting program active when power turned on / interrupted (runs for 7 mins)
Troubleshooting Steps
Safety precautions:
1) If you smell gas, leave the property immediately, do not operate electrical switches, do not smoke and call the gas emergency number. 2) Isolate the boiler electrical supply before touching wiring or internal parts. 3) Do not attempt to remove burner, fan, gas valves, or flue components — these are gas-safe activities. 4) Keep children and pets away while you inspect the appliance.
Initial homeowner checks:
1) Observe behaviour on power-up. If the boiler runs a single venting cycle of about 7 minutes and then starts normally, this is expected. 2) Note whether the 105 code appears only after power interruptions or every time the boiler is switched on. 3) Check the boiler display for any additional fault codes or a spanner/service symbol. 4) Check your mains supply and consumer unit for recent tripping or unstable supply; make sure the boiler has a stable supply and the isolator/fuse is OK.
Specific diagnostic and simple fixes you can safely try:
1) Reset correctly: use the boiler reset button once. Wait for the vent cycle to finish rather than repeatedly resetting. Repeated resetting can lock the boiler out. 2) Check the condensate pipe (common issue in cold weather): inspect the external condensate discharge pipe for frost or ice. If frozen, thaw the pipe by pouring warm (not boiling) water over the frozen section or wrapping with warm towels; then insulate the pipe. 3) Check the external flue terminal visually for obvious blockages (bird nests, leaves, snow) from the outside only. Remove light obstructions like leaves if safe to reach. Do not push objects into the flue. 4) Ensure vents and airways around the boiler are not blocked and that any outside terminal is clear of debris or vegetation.
When to call a professional / further diagnostic steps for an engineer:
1) Call a Gas Safe registered engineer if the 105 error repeats frequently, the venting cycle does not finish, other fault codes appear, or you see signs of poor combustion (soot, strong fuel smell). 2) The engineer will check the fan operation and PWM control, flue pressure and integrity, PCB behaviour and memory, condensate trap and drain, and any sensors related to safety interlocks. 3) If power interruptions are frequent, discuss installing a suitable UPS or addressing the supply issue to prevent repeated vent cycles and wear on components. 4) Any repairs involving the gas train, burner, fan internals, flue modifications, or PCB replacement must be performed by a qualified technician.
Final note: A single 7-minute vent cycle at power-up is generally normal. Persistent or abnormal behaviour, additional fault messages, or safety concerns require a Gas Safe engineer to diagnose and repair. Do not attempt gas-side or internal repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.