Main E133 Fault Code: Ignition Lockout Explained
The E133 fault code on a Main boiler indicates that the boiler attempted to light but failed to establish a flame, causing it to shut down into safety lockout. Rather than continuing to run unsafely, the boiler stops all heating and hot water output until the underlying problem is resolved. You may hear repeated clicking as the boiler tries — and fails — to ignite before the lockout triggers. This same fault code also appears on Potterton boilers, which share a very similar ignition and gas control architecture with Main.
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 Main E133 fault code mean?
The E133 fault code on a Main boiler indicates that the boiler attempted to light but failed to establish a flame, causing it to shut down into safety lockout. Rather than continuing to run unsafely, the boiler stops all heating and hot water output until the underlying problem is resolved. You may hear repeated clicking as the boiler tries — and fails — to ignite before the lockout triggers. This same fault code also appears on Potterton boilers, which share a very similar ignition and gas control architecture with Main.
Common causes
- Interrupted or absent gas supply Common
If gas isn't reaching the burner in sufficient quantity — due to a closed isolation valve, a prepayment meter that has run out of credit, or a disruption from the network — the boiler cannot ignite. Quickly check whether other gas appliances such as your hob or gas fire are working; if none of them are, the problem is upstream of the boiler entirely.
- Frozen condensate pipe Common
During cold snaps, the condensate pipe — which carries acidic water from the boiler to the drain — can freeze solid, especially if it runs along an outside wall. The blockage stops the boiler operating and can trigger an E133 lockout. A gurgling noise from the boiler before lockout is a tell-tale sign.
- Faulty or dirty ignition electrode or flame sensor Sometimes
The ignition electrode creates the spark to light the burner, and the flame sensor (ionisation probe) confirms the flame is present. If either component is worn, cracked, or coated in residue, the boiler may spark without the control board receiving a flame signal — causing a lockout even when gas is available.
- Gas valve fault Sometimes
The gas valve regulates fuel flow to the burner. If it sticks closed, operates sluggishly, or fails electrically, gas cannot reach the burner in the right quantity and ignition fails. This is an internal component that must be diagnosed and replaced by a Gas Safe engineer.
- Low system pressure Sometimes
Main boilers require system pressure in the region of 1 to 1.5 bar to operate correctly. If pressure has dropped significantly — visible on the boiler's pressure gauge — the boiler may refuse to ignite or lock out shortly after attempting to do so.
- Blocked or obstructed flue Sometimes
The flue removes combustion gases from the boiler to the outside. A bird nest, debris, or ice blocking the flue outlet prevents safe venting and causes the boiler to lock out as a precaution. The blockage is usually visible on inspection of the external flue terminal.
- PCB (printed circuit board) failure Rare
The PCB controls and coordinates every stage of the ignition sequence. If it develops a fault — due to age, a power surge, or component failure — it may send incorrect signals or fail to trigger ignition at all, producing an E133 lockout even when all other components are functioning normally.
How to fix it
- Check that your gas supply is working DIY safe
Before touching the boiler, confirm that other gas appliances in your home — your hob burners or a gas fire — are lighting normally. If they are not, contact your gas supplier or National Gas (0800 111 999) immediately. If you use a prepayment meter, check that it has sufficient credit. Do not attempt to reset the boiler if you suspect a gas supply problem.
- Inspect and thaw a frozen condensate pipe if the weather is cold DIY safe
Locate the condensate pipe — typically a white or grey plastic pipe (often 21.5 mm or 32 mm diameter) running from the boiler to an outside drain or soil stack. If it passes through or along an external wall, feel along its length for ice. If frozen, pour warm (not boiling) water along the pipe until it thaws, then dry the area. Boiling water can crack plastic pipework, so keep the temperature moderate.
- Check the system pressure and top up if needed DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop (usually a braided hose or inline valve beneath the boiler) to slowly add water until the gauge reaches 1.2–1.5 bar, then close the filling loop firmly. Consult your boiler's user guide if you are unsure how your particular filling loop works.
- Reset the boiler DIY safe
Once you have completed the checks above, press and hold the reset button (usually marked with a flame or reset symbol) for around three seconds. The boiler should attempt to re-ignite. If it fires successfully, monitor it for the next 30–60 minutes to confirm the fault does not return. If the E133 reappears promptly or after a short time, there is an underlying fault that resetting alone will not fix — do not continue resetting repeatedly without identifying the cause.
- Call a Gas Safe registered engineer Gas Safe engineer
If the E133 fault persists after completing all the homeowner checks above, or if it keeps returning after a reset, an engineer is required. A qualified Gas Safe engineer will test gas pressure at the boiler, inspect and test the ignition electrode, flame sensor, and associated wiring, check the gas valve operation, and examine the flue for blockages. Do not attempt to open the boiler casing or work on any internal gas components yourself — this is a legal requirement as well as a safety one.
Parts you may need
- Ignition electrode · from £25
- Flame sensor / ionisation probe · from £20
- Gas valve · from £120
- PCB (printed circuit board) · from £180
- Condensate pipe insulation lagging · from £8
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Main E133 fault code myself?
Some causes are safe for a homeowner to address: checking the gas supply, thawing a frozen condensate pipe, topping up system pressure, and resetting the boiler. These steps cost nothing and resolve a good proportion of E133 faults. Anything beyond that — testing or replacing the ignition electrode, flame sensor, gas valve, or PCB — must be carried out by a Gas Safe registered engineer. Working on gas components without Gas Safe registration is illegal and dangerous.
Why does my Main boiler keep showing E133 after I reset it?
If the fault returns quickly after resetting, the reset is simply clearing the lockout temporarily without addressing the root cause. The most common culprits for a recurring E133 are a failing ignition electrode or flame sensor, a faulty gas valve, or low gas pressure at the meter. A Gas Safe engineer will be able to run pressure and component tests to pinpoint the fault properly.
Could a frozen condensate pipe cause E133 on my Main boiler?
Yes — this is one of the most common causes of E133 during winter months. When the condensate pipe freezes, waste water backs up inside the boiler and the appliance locks out as a safety measure. Thawing the pipe with warm water and then resetting the boiler usually resolves it immediately. To reduce the risk in future, fit foam lagging over any exposed sections of condensate pipe that run outside or through unheated spaces.
How much does it cost to fix a Main E133 fault?
If the cause is a frozen condensate pipe or a gas supply issue, the fix can cost nothing at all — just your time. For component repairs carried out by a Gas Safe engineer, most people with this fault pay somewhere between £100 and £350 in total, covering both parts and labour. An ignition electrode or flame sensor replacement typically sits at the lower end of that range, while a gas valve replacement tends to be higher. In rare cases where the PCB has failed, costs can exceed £400 including parts and labour — if the boiler is older, it may be worth weighing a repair against the cost of a replacement.