Medium
AL 43 on Sime Mia 30

Your boiler is having trouble communicating with your smart thermostat or external heating controls, which means it may not know when to turn on or off.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Open Therm communication fault

From this model's manual
Verified against the Sime Mia 30 installation & service manual, page 45.
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

Check that your smart thermostat or wall controller is powered on and has working batteries.Ensure any wireless receiver boxes plugged into or near the boiler have power lights on.Turn the boiler off and back on at the main fused spur switch to refresh the connection.

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 AL 43: 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 fault codes

AL 06View →AL 40View →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

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 →