What E133 is
The boiler has run its ignition sequence, failed to establish or hold a flame, and locked itself out. Baxi's published error code list gives E133 as "Gas supply fault. Check the power supply, sensing electrode, spark electrode and cable." Separately, Baxi's homeowner guidance names E133 among the codes that appear when the condensate pipe has frozen, which in winter is the single most common trigger.
Nothing in either statement is about water. We are a leak company and this is not our job, so the useful thing we can do is tell you what to check and who to call.
The three checks you can make safely
- Is there gas at all? Try the hob or another gas appliance. If nothing else works either, the problem is upstream of the boiler. On a prepayment meter, check the credit, because an empty meter produces exactly this code.
- Is the condensate pipe frozen? Find the white or grey plastic pipe running outside from the base of the boiler. In cold weather it freezes and the boiler shuts down to avoid backing condensate up inside the case. Baxi's advice is to switch the boiler and its power off, warm the frozen section with a hot water bottle or heat pack rather than boiling water, then restart. Repeat if needed.
- Reset once, not repeatedly. One reset after clearing an obvious cause is reasonable. If it locks out again, stop. Each failed ignition cycle dumps unburnt gas into the combustion chamber before the boiler shuts down, and repeated resets wear the ignition and sensing electrodes.
When to stop and call a Gas Safe engineer
If gas is present, the condensate pipe is clear, and the boiler still locks out on E133, the remaining causes are all inside the appliance: the spark electrode, the flame sensing electrode, their leads, the gas valve, the fan or the PCB. Baxi's own note points the installer at the power supply, the sensing electrode, the spark electrode and the cable.
All of that is gas work. By law, anyone working on a gas appliance in the UK has to be on the Gas Safe Register, and you can check an engineer's registration and the appliances they are qualified for on the register's own site before they start.
If you smell gas at any point, do not reset anything. Turn the gas off at the emergency control valve by the meter, open windows, avoid switches and naked flames, and call the National Gas Emergency Service, whose number is published on the Gas Safe Register website.
The one overlap with what we do
There is a real case where a leak and an E133 turn up together, and it is worth naming because it gets missed.
A leak inside the boiler case, or water tracking down the flue or a pipe into it, can reach the ignition or sensing electrode and its connections. A damp electrode will not spark or will not read a flame, and the boiler locks out. The clue is not the code, it is what you can see: staining or corrosion under the case, a green or white bloom on pipework, water marks on the floor beneath the boiler, or a tide mark on the wall behind it. Baxi publishes a short guide to telling whether a boiler is leaking, and it is a sensible thing to read while you wait for the engineer.
The other overlap is simpler. If your boiler shows a pressure fault as well, or the gauge has been falling, that half is ours. E119 on a Baxi or Potterton is the code to read next, and the cross-brand list of codes that actually mean water loss sorts the rest.
Why the frozen condensate pipe keeps happening
If E133 arrives with the first hard frost every year, the pipe is the fault rather than the weather. The usual reasons are an external run that is too long, a diameter that is too small for the run, too little fall so condensate stands in the pipe, or an outlet exposed to wind. Thawing it gets you through the week. Re-routing it internally where possible, increasing the diameter of the external section, or lagging it properly stops it recurring, and that is ordinary plumbing rather than gas work.
What the engineer will look at, and in what order
Knowing the order helps you judge whether the person in your kitchen is working methodically or guessing. Gas pressure at the appliance comes first, standing and working, because a weak supply produces this code with nothing wrong inside the boiler. Then the spark: whether the electrode is firing at all, the gap it is set to, and the condition of its lead and connections. Then flame sensing, because an electrode coated in carbon cannot read a flame that is burning in front of it. Then the gas valve and the fan. The PCB is the last item on that list rather than the first, and a quote that opens with a new board and none of the earlier checks is a quote to replace the dearest component on a guess.
What not to pay for
Do not book leak detection for an E133 on its own. There is nothing for a leak engineer to find, and a company that sells you a detection visit on the strength of an ignition code is selling you a survey of a system that is not leaking.
Equally, do not accept a new boiler quote as the first answer. An electrode, a lead or a gas valve is a component-level repair on an appliance that is otherwise working. Get the fault diagnosed before anyone prices a replacement.
If the same visit turns up a heating leak as well, that part is ours: we charge a fixed hourly rate, agreed before we attend, with no find, no fee on the detection, and we carry out the repair and the making good rather than handing you a report and leaving.
