Main Eco Compact E135 fault code: internal boiler error

Seeing E135 on a Main Eco Compact can be frustrating because the wording is broad. Main’s current manufacturer error-code information lists E135 as “Internal error” for the model group that includes the Main Eco Compact Combi and Main Eco Compact System.

That description is deliberately general. It tells you the boiler’s controls have detected an internal condition they cannot accept, but it does not identify one failed part. E135 alone is not proof that the main circuit board, a sensor, a valve or any other component must be replaced.

What E135 means on a Main Eco Compact

The boiler continually checks information from its controls and safety systems. If those checks produce an internal error state, the boiler can display E135 and restrict or stop operation.

“Internal” refers to the boiler’s own control and monitoring system. It does not mean that you should open the casing to investigate. A closed boiler protects components and keeps the room separated from areas that must only be accessed by a suitably qualified engineer.

The code may appear with no heating, no hot water, or both. What the household notices can depend on whether the appliance is a combi or system model and on the point in its operating cycle when the error occurred.

What you may notice at home

Alongside E135, you might notice:

• the boiler does not start when heating is requested;
• radiators remain cold even though the programmer or thermostat is calling for heat;
• a combi boiler does not provide hot water;
• a system boiler stops heating the separate hot-water cylinder;
• the boiler begins a start-up sequence and then stops;
• the display returns to E135 after the appliance has been switched on normally.

These observations help describe the problem, but none of them pinpoints the failed component. Several internal conditions can produce similar symptoms.

Safe checks you can make

Keep any checks external and observational.

First, confirm the display really says E135. Photograph the screen if the code appears only briefly. Note whether it appeared during heating, hot-water demand, or start-up.

Check that the household controls are asking for heat in the usual way. Look at the thermostat and programmer without removing covers, changing installer settings or touching wiring.

You can also note whether other electrical equipment in the property is behaving normally. If the boiler has no display at all, that is different from a lit display showing E135 and is useful information for the engineer.

If you can see an external pressure gauge without opening or removing anything, record its reading. Do not assume that pressure caused E135, and do not add water solely because this code is present.

Finally, check for obvious water around the outside of the boiler and nearby visible pipework. Do not remove the case, reach behind panels or touch wet electrical equipment. If water is close to electrics, keep clear and arrange urgent professional help.

Should you reset the boiler?

Follow only the user instructions supplied for your exact model. If the manual permits one normal user reset, you can follow that published step once. Do not keep resetting the boiler if E135 returns.

Repeated resets can hide the pattern the engineer needs to see and will not repair an underlying internal fault. Record what happened before and after the single permitted reset. If you are uncertain which control is the user reset, leave the appliance alone.

Do not isolate and reconnect internal plugs, test electrical points, alter service settings or try combinations of buttons found in unofficial videos. Those actions are not homeowner checks.

Why E135 needs professional attention

The manufacturer description does not narrow E135 to one replaceable item. A qualified engineer needs to assess the appliance in the context of its exact model, service history and observed behaviour.

That assessment may include checks that require the case to be opened and the boiler to be tested safely. Those are not suitable DIY tasks. Gas appliances combine electricity, hot surfaces, water, combustion and safety controls, so guessing a part from the display can waste money and create risk.

Give the engineer:

• the full code and a photograph of it;
• the boiler model shown on accessible paperwork or the outside label;
• whether heating, hot water or both stopped;
• when the code first appeared;
• whether there was a recent power interruption, service or other work;
• what happened after any single user reset permitted by the manual.

This gives a clearer starting point without anyone interfering with the boiler.

Is E135 dangerous?

E135 means the boiler has recognised an internal error; the code itself does not prove that gas or carbon monoxide is escaping. Even so, never ignore signs of an emergency.

If you smell gas, do not use electrical switches or naked flames. Open doors and windows if it is safe, leave the property and contact the National Gas Emergency Service from outside on 0800 111 999.

If a carbon-monoxide alarm sounds, or anyone has symptoms such as headache, dizziness, nausea, weakness or confusion, get everyone into fresh air, seek medical advice and arrange an urgent Gas Safe check. Do not use the boiler again until it has been declared safe.

A water leak, scorching, smoke, unusual burning smell or repeated tripping of an electrical safety device also needs prompt attention. Keep away from the appliance and use the appropriate emergency service where necessary.

Can you keep using the boiler?

If the boiler has stopped and continues to show E135, do not try to force it to run. Arrange a repair.

If the code clears, record it rather than assuming the issue is permanently resolved. An intermittent internal error can return. If you notice unusual noises, odours, leaks or unstable operation, stop using the appliance and obtain professional help.

During cold weather, protect vulnerable people and use only safe temporary heating. Keep portable heaters clear of curtains and furniture, never cover them, and do not leave them running unattended.

What not to do

Do not:

• remove the front casing;
• test wiring, voltage, resistance or continuity;
• unplug or reseat internal connectors;
• replace a circuit board, sensor, valve or fuse by guesswork;
• enter installer or service menus;
• repeat resets to keep the boiler running;
• buy parts based only on E135.

The wording “internal error” is not a parts list. The correct remedy depends on evidence gathered safely from the individual appliance.

Quick Answers

What does Main Eco Compact E135 mean?

Main’s current error-code information defines E135 as “Internal error” for the group that includes Main Eco Compact Combi and System boilers.

Does E135 mean the circuit board has failed?

No. The code does not identify a particular component, so it is not proof of a failed circuit board or any other specific part.

Can I fix E135 myself?

Homeowner actions should be limited to safe external observations and any single reset explicitly allowed by the model’s user guide. Internal checks and repairs require a suitably qualified engineer.

Will E135 affect heating and hot water?

It can. A combi may lose heating and hot water, while a system boiler may stop providing heat to radiators or the hot-water cylinder. The exact symptom depends on the installation and fault state.

What should I tell the engineer?

Share the code, model, when it appeared, which services stopped, any recent power interruption or work, and what happened after any permitted user reset.