20 High

Main 20 Fault Code

The Main 20 fault code means: your boiler is struggling to control its internal gas supply, which means it has safely shut itself down to prevent any risk of incomplete combustion.. It appears on 7 Main models in our database.

Call an engineer soon

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

Technical description: Gas valve regulation fault
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 →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like 20: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

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

Main models that show the 20 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Main 20 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026