Common Boiler Error Codes Explained

What Are Boiler Error Codes?

Modern boilers display error codes on a small screen or via flashing lights when something goes wrong. These codes are designed to help engineers diagnose faults quickly — but they're useful for homeowners too.

Most Common Error Codes by Brand

Worcester Bosch

• E9 — Overheat cut-out triggered. The boiler got too hot and shut itself off for safety. Usually caused by low water pressure or a faulty pump.
• EA — Ignition failure. The boiler tried to light and couldn't. Check the gas supply first.
• E5 — Flow temperature too high. Often a sign of sludge build-up in the system.

Vaillant

• F.22 — Low water pressure. Re-pressurise the system to 1–1.5 bar.
• F.28 / F.29 — Ignition lockout. Gas supply issue or blocked flue.
• F.75 — Pressure sensor or pump fault.

Ideal

• F1 — Low water pressure. Top up via the filling loop.
• F2 — Flame loss during operation. Possibly a faulty gas valve.
• L2 — Flame detection issue.

What Should You Do?

1. Note the code and look it up in your boiler manual.
2. Check the basics first — gas supply, water pressure, and that the flue isn't blocked.
3. Reset the boiler — most codes clear with a reset. If it comes back within an hour, call an engineer.
4. Don't ignore recurring faults — repeated lockouts are a sign of an underlying issue.

When It's Time to Call Aphex Heating

If the code keeps returning after a reset, or if you're unsure what it means, our Gas Safe engineers can diagnose and fix the fault the same day in most cases across St Albans.