BoilerFaultCodesWhat the manual says
Manufacturer manualFlow Thermistor Fault
From this model's manual
Verified against the Zanussi Ultra Plus Combi installation & service manual.
What causes this fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water being sent to your taps and radiators. The most common reason is that the sensor has physically failed or developed an electrical short, though loose or damaged internal wiring can also prevent the boiler from reading the temperature correctly.
Engineer required
This is a high severity fault. Stop using the boiler and call 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
Get a quote →
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Affiliate links. We may earn a commission at no extra cost to you.