Atag Fault Code 133: No Flame After 5 Ignition Attempts
Fault code 133 means your Atag boiler has tried to light itself five times in a row and failed to detect a stable flame on any attempt. As a safety precaution the boiler then locks out, shutting down completely until the underlying problem is identified and cleared. The fault points to a breakdown somewhere in the ignition or flame-detection process — or to something preventing gas from reaching the burner in the first place.
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 133 fault code mean?
Fault code 133 means your Atag boiler has tried to light itself five times in a row and failed to detect a stable flame on any attempt. As a safety precaution the boiler then locks out, shutting down completely until the underlying problem is identified and cleared. The fault points to a breakdown somewhere in the ignition or flame-detection process — or to something preventing gas from reaching the burner in the first place.
Common causes
- Frozen condensate pipe Common
In cold weather the condensate pipe — which carries acidic waste water away from the boiler — can freeze solid, especially along any section that runs outside. When the pipe blocks, the boiler cannot safely dispose of combustion by-products and shuts down. Atag themselves specifically list a frozen condensate pipe as a primary trigger for fault code 133 during winter months.
- Interrupted gas supply Common
If the gas supply to the boiler is disrupted — whether because a meter or supply valve has been inadvertently closed, because of a wider street-level interruption, or because of a pressure drop — the burner will have nothing to ignite and will fail every attempt.
- Worn or fouled ignition electrodes and flame sensor Sometimes
The ignition electrodes generate the spark that lights the burner, and the flame sensor (ionisation probe) confirms a flame is present. If either component is coated in deposits from normal combustion, cracked, or has deteriorated with age, the boiler may spark without successfully lighting or may fail to recognise a flame even when one exists.
- Faulty gas valve Sometimes
The gas valve controls the precise flow of gas to the burner. If it is failing electrically or mechanically, it may not open fully or at the right moment, starving the burner of gas during the ignition sequence. Replacement requires a Gas Safe registered engineer.
- Blocked or contaminated burner Rare
Debris, corrosion products, or a build-up of deposits inside the burner can disrupt the gas-air mixture and prevent reliable ignition. This is more likely in boilers that have not been serviced regularly.
How to fix it
- Check whether other gas appliances in your home are working DIY safe
Try a gas hob or check with your gas supplier whether there is a local interruption. If no gas appliances are working, contact your gas supplier — there is nothing else you can do until supply is restored.
- Inspect the condensate pipe for signs of freezing DIY safe
Locate the plastic condensate pipe — usually a white or grey 21.5 mm pipe that exits through an external wall or runs outdoors. In cold weather, look for frost or ice on the outside of the pipe. A gurgling noise from the boiler before lockout is another tell-tale sign.
- Thaw a frozen condensate pipe carefully DIY safe
Gently warm the pipe with a hot-water bottle, a microwaveable heat pack, or cloths soaked in warm (not boiling) water. Work along the pipe starting closest to the blockage — typically the lowest outdoor section or any exposed bend. Never use a heat gun or open flame. Once the pipe feels clear and water can be heard flowing, proceed to the reset step.
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's control panel as described in your user manual — usually for around three seconds until the display changes. Allow the boiler a full minute to attempt ignition before judging whether it has cleared. Avoid resetting more than two or three times; repeated unsuccessful resets can indicate a more serious fault and excessive resets are not recommended.
- Call a Gas Safe registered engineer if the fault returns or does not clear Gas Safe engineer
If the boiler locks out again after resetting, or if there was no frozen pipe and the gas supply is confirmed on, the fault lies with an internal component — ignition electrodes, flame sensor, gas valve, or burner. All of these require investigation and repair by a Gas Safe registered engineer. Do not attempt to access or adjust any internal gas components yourself.
Parts you may need
- Ignition electrode set (pair) · from £35
- Flame sensor / ionisation probe · from £25
- Ignition lead set · from £20
- Gas valve (Honeywell or equivalent) · from £120
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 133 myself?
Possibly, if the cause is a frozen condensate pipe — thawing it and resetting the boiler is straightforward and completely safe for a homeowner. Confirming your gas supply is on is equally easy to check. Beyond those two things, the fault involves gas components that must only be worked on by a Gas Safe registered engineer. Attempting DIY repairs on electrodes, the gas valve, or the burner is illegal and potentially dangerous.
Why does my Atag boiler keep showing fault code 133 even after I reset it?
Repeated lockouts almost always mean the root cause has not been resolved. If thawing the condensate pipe and resetting once or twice has not cleared it, the fault is likely inside the boiler — a worn ignition electrode, a failing flame sensor, or a gas valve that is not opening correctly. At this point you need an engineer to diagnose and fix the specific component rather than continuing to reset.
How much will it cost to repair an Atag boiler showing fault code 133?
Most people with this fault pay somewhere between £120 and £350 all in, depending on which part needs replacing. A call-out and diagnosis alone typically runs £75–£150. Replacing ignition electrodes or the flame sensor tends to sit at the lower end of that range, while a gas valve replacement is usually the most expensive common repair at around £180–£250 including labour. In rare cases where a PCB or heat exchanger is implicated, costs can be considerably higher — if that is quoted on an older boiler, it may be worth comparing against the cost of a new installation.
How can I stop my Atag boiler freezing up and triggering fault code 133 every winter?
The most effective step is to insulate any section of condensate pipe that runs outside or through an unheated space such as a garage or loft. Foam pipe lagging is inexpensive and straightforward to fit. Keeping your heating on a low frost-protection setting during very cold spells (rather than turning it off completely) also helps keep the condensate flowing. An annual boiler service will keep the ignition components and burner clean, reducing the risk of fault code 133 from those causes.