Atag Boiler Fault Code 105
Fault code 105 on an Atag boiler points to a problem with the condensate drainage system — typically a blocked or frozen condensate pipe that is preventing the boiler from expelling acidic waste water safely. On some Atag models the related code C105 can also indicate a failure to ignite or establish a flame, which may follow on from a condensate blockage once the pipe is cleared and air has entered the gas line. In either case the boiler shuts itself down as a protective measure until the underlying issue is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the ATAG 105 fault code mean?
Fault code 105 on an Atag boiler points to a problem with the condensate drainage system — typically a blocked or frozen condensate pipe that is preventing the boiler from expelling acidic waste water safely. On some Atag models the related code C105 can also indicate a failure to ignite or establish a flame, which may follow on from a condensate blockage once the pipe is cleared and air has entered the gas line. In either case the boiler shuts itself down as a protective measure until the underlying issue is resolved.
Common causes
- Frozen condensate pipe Common
During cold snaps the water sitting in an external or poorly insulated condensate pipe can freeze and expand, forming an ice plug that completely blocks condensate flow. This is by far the most frequent trigger for fault code 105 in winter months and is something a homeowner can usually resolve without calling an engineer.
- Debris or sludge blockage Sometimes
Over time, limescale, dirt and biological sludge can accumulate inside the condensate pipe, gradually narrowing the bore until flow is restricted or cut off entirely. This type of blockage tends to develop slowly and may not be linked to cold weather.
- Incorrectly installed or sagging pipe Sometimes
If the condensate pipe was fitted without a sufficient fall towards the drain, or if pipe clips have failed and allowed it to sag, condensate pools in the low spots rather than draining away. The standing water can freeze more readily or simply back up into the boiler.
- Ignition or flame-detection failure (C105) Rare
Where the fault is displayed as C105, the boiler has attempted to light but failed to detect a stable flame. This can be caused by air trapped in the gas line after a previous shutdown, a fouled or damaged ignition electrode, a dirty flame-sensing probe, or an interruption to the gas supply.
How to fix it
- Check the gas supply is on DIY safe
Confirm that the gas isolation valve serving the boiler is in the open position (handle in line with the pipe). Check that other gas appliances in the property — a hob or gas fire — are working normally. If no gas appliances are working, contact your gas supplier rather than attempting a boiler reset.
- Reset the boiler DIY safe
Press and hold the reset button on the Atag control panel as described in your user guide, then release it and allow the boiler up to two minutes to restart. If the fault code clears and the boiler fires successfully, monitor it over the next hour to see whether the error returns. Do not attempt more than two or three resets in total — repeated failed resets will not fix an underlying fault and can mask important diagnostic information for an engineer.
- Inspect the condensate pipe for freezing or obvious blockage DIY safe
Trace the white or grey plastic condensate pipe from the boiler to where it exits the property and terminates at a drain. Feel along its length for sections that are noticeably colder than others, look for visible ice through translucent pipework, or listen for gurgling that stops abruptly. On a cold day, a frozen external section is almost certainly the culprit.
- Thaw a frozen condensate pipe DIY safe
Boil the kettle, then allow the water to cool for at least 15 minutes so it is warm rather than scalding — using boiling water risks cracking the plastic pipe. Slowly pour the warm water along the frozen section, working from the boiler end outward. Repeat as needed until you hear condensate flowing freely into the drain. Once thawed, reset the boiler again. Afterwards, wrap the external section of pipe in foam lagging secured with waterproof tape to reduce the chance of it freezing again.
- Top up boiler pressure if it has dropped DIY safe
A prolonged shutdown can sometimes cause system pressure to fall. Check the pressure gauge on the boiler — it should sit between 1.0 and 1.5 bar when the heating is cold. If it reads below 1 bar, use the filling loop (the braided flexible hose beneath the boiler with one or two isolation valves) to slowly admit mains water until the gauge reaches around 1.2 bar, then close the filling-loop valves fully.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler continues to display fault code 105 or C105 after the steps above, the cause is likely internal — a failed ignition electrode, a contaminated flame sensor, a faulty PCB, or a condensate system fault that requires dismantling pipework inside the appliance. All of these tasks must be carried out by a Gas Safe registered engineer. When you call, let them know the exact code displayed (105 or C105) and describe what you have already tried, so they can bring the most likely replacement parts.
Parts you may need
- Ignition electrode · from £35
- Flame sensor (ionisation probe) · from £45
- Condensate trap and non-return valve · from £25
- External condensate pipe lagging kit · from £12
- Condensate pump (where gravity drain is not possible) · from £85
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 105 myself?
If the cause is a frozen condensate pipe, yes — thawing it with warm (not boiling) water and then resetting the boiler is a straightforward DIY task that costs nothing and works in the majority of winter cases. Checking that your gas supply is on and topping up system pressure are also safe for homeowners. However, if the fault persists after these steps, the repair will involve internal boiler components and must be handled by a Gas Safe registered engineer.
Why does my Atag boiler show fault 105 only in cold weather?
Cold weather is almost always the culprit. The condensate pipe carries acidic waste water from the boiler to an external drain, and any section of that pipe exposed to outside temperatures can freeze when it drops below 0 °C. The resulting ice plug prevents drainage, the boiler detects the problem and locks out for safety, and fault code 105 appears. Insulating the external pipe with foam lagging is the most effective way to prevent it happening again.
What is the difference between fault code 105 and C105 on an Atag boiler?
Fault code 105 generally points to a condensate drainage fault, whereas C105 on some Atag models specifically flags an ignition or flame-detection failure. In practice the two can be related: a condensate blockage shuts the boiler down, and once the blockage is cleared, air in the gas line can temporarily prevent the burner from lighting, triggering C105. An engineer can distinguish between the two using boiler diagnostics and will check both the condensate system and the ignition components if C105 is displayed.
How much should I expect to pay for an engineer to fix fault code 105?
For a straightforward condensate blockage or pipe repair the typical cost is £120–£350 including labour and any parts. If the fault is C105 related to ignition components such as an electrode or flame sensor, most repairs fall within the same range. More involved work such as replacing a PCB or internal heat exchanger can cost considerably more — often £400–£700 or above — but those scenarios are uncommon for this fault code. Getting two or three quotes from Gas Safe engineers is always worthwhile for any repair likely to exceed £200.