Medium
F4 on Protherm 100EC

Your boiler is struggling to accurately measure your hot water temperature, which usually means it will temporarily stop providing hot water until the sensor is fixed.

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

Domestic hot water thermistor faulty

From this model's manual
Verified against the Protherm/Protherm 100ec 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.

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 F4: 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

F3View →F5View →

All 5 fault codes on the 100EC

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

F0F1F2F3

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 →