Low
A.20 on Viessmann Vitodens 100-W WB1A

Your boiler's internal calendar or clock is incorrectly set, which is preventing it from tracking when its next maintenance check is due.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

Service interval could not be activated. Measure: Check the time and date settings.

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Locate the settings menu on your boiler's control panelNavigate to the time and date configuration screenCorrect the current time and date to match your local UK timezoneSave the settings and restart the boiler to clear the prompt

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like A.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.

Related guides you might find useful

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →