Low
UF on Zanussi Ultra Plus Combi

Your boiler is currently set to a professional test mode for servicing and is not in its normal operating state.

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

Fault history mode diagnostic code

From this model's manual
Verified against the Zanussi Ultra Plus Combi 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.

What causes this fault?

This code appears when the boiler has been manually put into a high-performance test mode by a heating engineer during a service or inspection. It occurs because the boiler is bypasssing its normal temperature controls to run the fan at maximum speed for flue gas analysis. It is not a mechanical failure but rather a status indicator that the unit is being tested.

DIY fix possible

Locate the 'Mode' or 'Reset' knob/button on the front panel.Turn the selector knob away from the 'Service' or 'Spanner' icon back to 'Auto' or 'On'.Press the 'Reset' button once if the code persists after changing the mode.

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 UF: 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

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 →

New boiler cost 2026

Realistic UK prices for a swap.

Read →