High
L5 on Zanussi Ultra Plus Combi

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back into the unit, causing it to shut down for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Return Thermistor Fault

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 fault occurs when the boiler's control board receives an impossible or inconsistent reading from the sensor that monitors water returning from your radiators. The most common reason is that the sensor has physically failed or developed a short circuit, but it can also be triggered by damaged internal wiring or a fault with the main circuit board.

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.

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