Main E118 fault code explained

E118 on a Main Eco Compact boiler means the heating-system water pressure is low. Baxi’s official fault-code table lists E118 for the Main Eco Compact Combi and Main Eco Compact System ranges and defines it as water pressure below 0.5 bar.

The pressure gauge can help you confirm what the boiler is reporting, but low pressure is a symptom rather than a complete diagnosis. If the pressure keeps falling, the code returns or you cannot repressurise the system exactly as the user guide describes, arrange an engineer visit.

What does Main fault code E118 mean?

The boiler has detected system pressure below the level it needs for normal operation. The display may show E118 and the boiler may stop providing heating or hot water to protect the system.

For the Main Eco Compact Combi and System models, the manufacturer’s published threshold is below 0.5 bar. Other Main ranges can use different displays or instructions, so do not assume E118 applies to every boiler carrying the Main name.

Check the pressure gauge

Look at the gauge without removing any covers. The Main Eco Compact is designed with an accessible pressure gauge, so you should be able to read it from the outside of the appliance.

Compare the reading with the normal cold-pressure range printed in your exact user guide. If it is below 0.5 bar and the display shows E118, the gauge and code are telling the same story. If the gauge appears normal but E118 remains, photograph both displays and arrange professional diagnosis rather than trying to override the fault.

Can I repressurise the boiler myself?

Many sealed heating systems allow the householder to restore pressure using the approved filling arrangement. The controls and sequence vary, however, so only follow the instructions for your exact Main Eco Compact model.

• Use the official user guide for the model on your boiler.
• Keep the boiler casing closed.
• Watch the gauge continuously and stop at the pressure stated in the guide.
• Close the filling controls fully when the correct pressure is reached.
• Do not continue if the controls are missing, damaged, leaking or unfamiliar.

Never guess which valve to turn. Do not overfill the system and do not remove the casing to reach internal components. If you are unsure, leave the boiler alone and book an engineer.

Why has the pressure dropped?

A small pressure loss can occur over time, particularly after radiators have been bled, but repeated loss needs investigation. Possible causes include a visible leak from the heating system, a problem with a valve or radiator, or an internal component that needs testing. The E118 code cannot identify which of these is responsible.

Check only the visible parts of the system: radiators, accessible pipe joints and the area beneath the boiler. Do not dismantle valves or tighten boiler components. Water marks, damp patches or a gauge that falls again after repressurising are useful clues for the engineer.

Why does E118 keep coming back?

If the code clears after pressure is restored but returns later, the system is losing pressure or the pressure is not being measured correctly. Repeatedly topping up introduces fresh water into the system and can mask the cause, so it should not become a routine solution.

Record the cold pressure after repressurising and check it again later. A photograph of the gauge is often more helpful than relying on memory.

When should I call an engineer?

Arrange a visit if:

• the pressure will not rise;
• you cannot identify or safely operate the filling controls;
• water leaks while you are filling;
• the pressure rises too high;
• E118 returns after the pressure has been restored;
• the gauge reading and display do not agree; or
• there is visible water around the boiler or heating system.

If you smell gas or a carbon-monoxide alarm sounds, do not attempt to repressurise or reset the boiler. Follow the emergency guidance from the Gas Safe Register and the gas emergency service.

What should I have ready when booking?

Take a photo of the E118 display and the pressure gauge, note the exact model and say whether the pressure has been restored before. Mention any visible leak, recent radiator bleeding or work on the heating system. These details help the engineer prepare without you having to open the boiler.