What is F23 fault code on Vaillant boiler

Chester, November 2011

check wall thermstat batteries this caused my f23 code

peter thomas, November 2021

Vaillant eco tec plus How can I get a F23 fault code when I am not using central heating ?
I have cleaned the heat exchanger (not very dirty) changed both sensors (on the exchanger and hot water flow)
the original fault was the pressure vessel was full of water, so changed it. I have a good flow rate and the pump works fine . BUT my hot water goes from hot , to cold, to hot , to cold then nothing but F23 fault

arthur, June 2016

Water temprature difference between flow and return ntcs too large.

Possible causes are:

Water pressure too low:

Pump running too slowly:

Faulty flow or return ntc:

Wiring to ntcs has been reversed:

Carefree, November 2011