Overview
Error code 133 on an ATAG boiler means the burner failed to establish and detect a flame after five ignition attempts and the boiler has gone into lockout. The boiler’s safety controls stop further ignition attempts to prevent unsafe conditions. Common causes are a lack of gas reaching the burner, a fault with the ignition/ flame-detection system (igniter or flame sensor), a fan or air supply problem, or an obstruction/blockage in the condensate discharge (often a frozen condensate pipe in cold weather) that interferes with the boiler’s normal sequence. Severity is medium to high: the boiler shutting down is a designed safety response, so there is no immediate danger from the boiler itself while it is locked out, but ignition faults involve gas and combustion so they require care. If you smell gas, treat that as an emergency — leave the building and call the gas emergency number immediately. Some simple checks are safe for a competent homeowner, but internal ignition or gas-valve work and electrical testing must be left to a Gas Safe registered engineer.
Possible Cause: No flame after 5 ignition attempts. This could be down to your condensate drain pipe freezing.
Troubleshooting Steps
Safety first
- If you smell gas: do not operate electrical switches, do not stay in the property. Get everyone out and call the national gas emergency number immediately.
- If you are unsure or uncomfortable with any steps below, stop and call a Gas Safe engineer. Do not attempt to access internal boiler components or tamper with gas valves.
Initial homeowner checks (safe, simple steps)
1) Note the code and conditions: write down the error code (133), any spanner/service symbol, and the time it happened. This helps an engineer.
2) Check other gas appliances: confirm the gas supply is on by briefly testing another gas appliance (hob). If no other gas appliance works, contact your gas supplier.
3) Power and reset: check the boiler has mains power (display lit). If it’s safe, try a single reset using the boiler reset button as described in your manual. Do not repeatedly reset more than twice if it does not clear.
4) Condensate pipe check (common winter cause): locate the external condensate discharge (a small plastic pipe from the boiler to outside). In freezing weather this can ice up and block. If it looks frozen or blocked, gently thaw it using warm (not boiling) water poured along the outside run or a warm cloth. Do not use a naked flame or steam cleaner. After thawing, run the boiler/reset to see if it clears the fault.
5) External flue/air inlet: visually check for obvious blockages (bird-nest, leaves, snow) around the flue or air inlet and remove small visible debris only if safe to do so from ground level.
6) Check for audible fan/pump: when the boiler attempts to start, you should hear the fan and ignition sequence. If there is complete silence it can indicate no mains, a tripped fuse, or a failed fan/PCB — call an engineer for further checks.
If the fault remains after the above
- Leave the boiler turned off (follow manufacturer guidance in your manual). Do not attempt to access or replace ignition electrodes, gas valves, flame sensors, the PCB, or other sealed components. These items are hazardous and must be handled by a Gas Safe engineer.
- Call a qualified Gas Safe registered heating engineer and give them: the boiler model, error code 133, details of whether thawing the condensate pipe was required, whether other gas appliances work, and whether the boiler responded to a reset. This speeds diagnosis.
What the engineer will check (for your information)
- Confirm gas supply and measure gas pressure to the boiler.
- Test ignition transformer/igniter and flame detection sensor and replace if faulty.
- Inspect and test the gas valve and burner for correct operation and cleanliness.
- Check fan operation and flue/air-pressure switches and related wiring.
- Inspect condensate trap and internal condensate routing for blockages or frozen sections and clear/replace as required.
- Check PCB and control wiring if intermittent faults or communication errors are present.
When to call urgently
- You can’t restore heating/hot water and cannot identify a clear, safe reason (like a thawed condensate pipe) for the lockout.
- The boiler repeatedly locks out after resets.
- You smell gas or suspect a leak.
Follow-up and prevention
- If the cause was a frozen condensate pipe, insulate the external pipe or fit a condensate trace heater/sleeve and consider routing to an internal drain where possible to reduce reoccurrence.
- Keep annual servicing by a Gas Safe engineer to reduce ignition and sensor faults.
Remember: simple external checks and thawing a frozen condensate pipe are often enough to clear a 133 fault, but ignition, gas-supply and electrical faults require a qualified Gas Safe engineer. Do not attempt internal repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Atag ATAG Boiler.