Medium
F.91 on Glow-worm Energy7 Regular

Your boiler is having trouble communicating with the temperature sensor in your hot water cylinder, which means it cannot properly monitor or heat your stored water.

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

Short circuit: Cylinder temperature sensor

Shared across the Energy7 range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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 →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like F.91: 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

F.92View →F.93View →

All 79 fault codes on the Energy7 Regular

Seeing a different code? Every code this model can display, in one place.

Connectiond.02d.94F.00F.01F.10F.11F.13F.20F.23F.24F.25F.27F.28F.29F.32F.49F.61F.62F.63F.64F.65F.67F.68F.70F.71F.72F.77F.78F.83F.84F.85F.86F.87F.88F.97P.01P.02P.03P.04P.05P.06P.07P.10S.0S.01S.02S.03S.04S.05S.06S.07S.08S.20S.21S.22S.23S.24S.25S.26S.27S.28S.30S.31S.32S.34S.39S.42S.46S.53S.54S.88S.91S.96S.98S.99S.108S.109

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on a combi.

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 →