BoilerFaultCodes.com
F.66 Medium Lockout

Your boiler's main control panel is having trouble communicating with the internal display screen, meaning you cannot see or adjust your heating settings.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Display fault | Display defect; fault in the connection to the display; electronics defect

What causes this fault?

This fault occurs when the boiler's 'brain' can no longer talk to the screen on the front of the unit. The most common reason is a loose or damaged data cable inside the boiler, or a component failure on one of the circuit boards caused by age and heat. Because these parts cannot share information, the boiler stops working to prevent you from running it without any visual control.

Typical repair cost £180-350 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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 F.66 — 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.

Vaillant Thermocompact 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