BoilerFaultCodes.com
Red flashing light High Lockout

Your boiler has encountered a general operation error and needs to be restarted to resume heating and hot water.

Call an engineer soon

Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Fault occurred. Reset boiler: Turn the On/Off switch to ( 0 ), wait for five seconds. Turn the On/Off switch to ( I ) the boiler is reset.

What causes this fault?

This fault occurs when the boiler's internal computer detects an interruption in the normal operating sequence, causing it to shut down for safety. The most common reasons include a failure to sense a flame correctly or a momentary communication error between the electronic components. If the reset doesn't work, it usually indicates that a part like the ignition lead or the main circuit board is failing to send the correct signals.

Typical repair cost £0-120 (including labour)

DIY fix possible

Locate the On/Off switch on the boiler control panelTurn the switch to the '0' positionWait for five secondsTurn the switch back to the 'I' position to reset the unit

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Protect yourself from future repair bills

A boiler cover plan would cover faults like Red flashing light — no call-out fees, no surprises.

Top rated
OpenFix
Boiler & heating care plans
Get a quote →
Hometree
from £12.95/month
Get a quote →
HomeServe
from £8.00/month
Get a quote →

Affiliate links — we may earn a commission at no extra cost to you.

Glow-worm 18si 24ci is discontinued — time for a new one?

Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.

UK's #1 online installer
Boxt
Fixed-price online quotes. Next-day installation.
Get a free quote →
Heatable
Instant online boiler quotes from £1,499.
Get a free quote →
iHeat
Fast installation. Price match guarantee.
Get a free quote →

Affiliate links — we may earn a commission at no extra cost to you.

Related fault codes