He just wants more money from you - get shot of him.
Why?
Your Central Heating is Good
Your Hot Water is Good
There are no Error Codes
Clearly the circuit board (PCB) is working
He is a charlatan - a swindler out to rob you.
All boiler engineers know full-well that the control board just opens and closes the gas valves - they do not regulate the flow other than on/off.
You do not actually say what your problem is - all you mention is the problem the boiler engineer claims.
Dismiss him from your service.
Good Luck...
September 2015