Just got my boiler fixed after three days without reliable hot water.
We had two engineers in over two days, and a total of 4 hours where neither could figure out the problem. Then we called in the manufacturer’s engineer (Vaillant)
The outlet sensor was faulty and was registering a temperature of 999 deg C. The boiler kept flaring up then tripping the safety cutoff as a result.
5 minutes, and one replacement part later and everything was fixed.