Main E135 Fault Code: Causes, Fixes & Repair Costs
What does the Main E135 fault code mean?
The E135 code on Main boilers indicates that the boiler has shut itself down — locked out — after detecting a problem with its internal gas supply or after failing to achieve ignition. Rather than pointing to an issue with your property's external gas supply, this code suggests something inside the boiler is preventing it from lighting correctly. The boiler enters lockout as a safety precaution to stop any potentially unsafe condition from continuing. Because this code is shared across the wider Baxi group platform, you may see E135 on Main, Baxi, and Potterton boilers — they all use the same underlying fault code system.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Faulty or Seized Gas Valve Common
The gas valve regulates how much gas reaches the burner and works in close communication with the PCB. Over time the valve's internal mechanism can seize or stick closed, starving the burner of gas and preventing ignition. The boiler will try to light, fail, and lock out after a set number of attempts. A seized valve is one of the most frequent findings when engineers investigate a persistent E135.
- Ignition Electrode or Lead Failure Common
The ignition electrode produces the spark that lights the gas, while the lead carries the electrical signal to create it. Either component can develop cracks, carbon fouling, or corroded terminals over time. A degraded electrode may not spark reliably, and a damaged lead may not deliver enough energy — both result in failed ignition attempts that ultimately trigger an E135 lockout.
- PCB (Printed Circuit Board) Fault Sometimes
The PCB is effectively the boiler's control centre, coordinating the gas valve, ignition system, and sensors. If the PCB develops a fault — whether from age, moisture ingress, or a component failure — it may send incorrect signals to the gas valve or ignition system, causing a lockout even when the physical components themselves are working properly. PCB faults are harder to diagnose and usually require a trained engineer with test equipment.
- Interrupted External Gas Supply or Empty Prepayment Meter Sometimes
Although the E135 primarily flags internal boiler issues, a complete loss of gas reaching the boiler will produce the same lockout behaviour. If you are on a prepayment meter, check it has sufficient credit. You can also verify gas is flowing by testing another gas appliance such as a hob. If there is no gas at any appliance, contact your gas supplier rather than an engineer.
- Blocked Flue or Internal Sludge Buildup Rare
A partially blocked flue can prevent combustion gases from clearing, causing safety sensors to shut the boiler down before ignition completes. Similarly, sludge or debris accumulation within the heat exchanger or pipework can disrupt normal operation to the point where the boiler cannot sustain a flame and locks out.
How to fix it
- Check other gas appliances in your home DIY safe
Turn on a gas hob ring or check another gas appliance. If nothing is working, the issue is with the gas supply to your property — contact your gas supplier or call the National Gas Emergency line on 0800 111 999. There is no point resetting the boiler until gas flow is restored.
- Check your prepayment meter has credit DIY safe
If you are on a pay-as-you-go gas meter, verify it has not run out of credit. Top it up if necessary and allow a minute or two for gas pressure to restore before attempting a reset.
- Reset the boiler — once or twice only DIY safe
On most Main boilers, turn the selector dial to the reset position (marked 'R') and hold it for around five seconds before releasing. The boiler will attempt to re-ignite. If it fires up and runs normally, monitor it over the next day or two. If the E135 returns quickly or the boiler will not light after two reset attempts, stop resetting — repeated resets without fixing the underlying cause will not help and could mask a developing fault.
- Do not attempt further resets if the fault persists DIY safe
A reset is not a repair. If the E135 keeps coming back, the boiler is telling you something is wrong internally. Continuing to reset it will not resolve the root cause and could, in some circumstances, stress components further. At this point the job belongs with a qualified engineer.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
An engineer will carry out a full inspection covering gas valve operation, ignition electrode and lead condition, PCB function, gas pressure at the boiler, and flue integrity. They will identify which component has failed and carry out the repair safely. Because the E135 involves the boiler's internal gas circuit and safety systems, all repair work must be performed by someone on the Gas Safe Register — this is a legal requirement, not just a recommendation.
Parts you may need
- Gas Valve · from £120
- Ignition Electrode · from £25
- Ignition Lead · from £20
- PCB (Printed Circuit Board) · from £220
- Flame Sensor / Ionisation Probe · from £30
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Main E135 fault myself?
The initial checks — confirming your gas supply is on, topping up a prepayment meter, and resetting the boiler — are perfectly reasonable things to do yourself. Beyond that, no. The E135 involves the boiler's internal gas system, and any work on the gas valve, ignition components, PCB, or flue must be carried out by a Gas Safe registered engineer. Attempting gas-related repairs without registration is illegal and dangerous.
Why does my Main boiler keep showing E135 even after I reset it?
If the fault returns within a short time of resetting, there is an underlying component problem that the reset cannot fix. The most common culprits are a gas valve that is starting to seize, a worn ignition electrode, or a developing PCB fault. Each reset simply asks the boiler to try again — it will keep locking out until the faulty part is repaired or replaced. Book a Gas Safe engineer rather than continuing to reset.
How much does it typically cost to repair an E135 fault in the UK?
Most people pay somewhere between £150 and £380 all in, depending on which component has failed. Replacing an ignition electrode or lead sits at the lower end — often £120 to £180 including labour — while a gas valve replacement typically costs £180 to £300. In cases where the PCB is at fault, costs can be higher; PCB replacement can reach £400 to £550 depending on the model, though some PCBs can be repaired rather than replaced, which brings costs down. Always get a written quote before work begins.
Does the E135 mean my boiler is dangerous?
Not necessarily. A lockout is a safety feature — the boiler has detected a problem and shut itself down to prevent an unsafe situation from developing. Your home is not in immediate danger when the boiler is in lockout. However, you should not attempt to bypass or repeatedly override the lockout, and you should arrange for a Gas Safe engineer to investigate the fault promptly, particularly if it is keeping you without heating or hot water.