Ecomax VU186EH boiler keeps shutting off (f.21 fault code)

[IDN101]

Last Thursday morning our Valliant Ecomax vU186EH boiler just stopped working with F.21 showing on the display.

Pressing the reset switch would NOT reset it- it was dead to the world.

Valliant engineer came out this morning- replaced the PCB- boiler fired and stayed on. Yet a few hours later, it shut down again with F.21 showing. This time Reset switch DOES reset the boiler and it fires up- BUT only works for (say) 10 mins and then it shuts down again- f.21.

Phoned Valliant tonight to let them now. Engineer coming back tomorrow.

Any thoughts as to what's wrong with it???

many thanks,
ian

 

[MZR]

Fault code F21 refers to general "non-ignition", but isn't too clear as to what might cause it.