PHE sensor failure and on....

As a newbee i have come to you as I have reached an inconclusion. The symptom of this 18kw was fluctuatiing hot water output temp from cold to well above the regulated 55C. The ACB diagnosed a faulty PHE sensor. I got one froe SEGEL and replaced - no difference. Running a reboot of the ACB didn't solve the problem but I noticed that the pump adjacent to the PHE wasn't running and during test a buzzing came from the ACB _ Iassume this is a relay and the pump isn't working. At the same time, of the two red LEDs on the fan control board, the right hand one is permanently flashing.
Have I diagnosed the correct fault? Do I have a faulty pump or zummat else?
TKU in antic.

p.s. i'm not in the trade but two plumbers have so far not been able to improve things and I do like a challenge - but I am getting a bit fed up.
Rob

Is it maybe the DHW sensor rather than the PHE sensor? Does the ACB error message show the same for both faults?
Rob

I solved the problem - mostly. The last plumber had replaced the DHW sensor rather than the the PHE sensor. To be fair he didn't have the MIs. I replaced the PHE sensor and the temperature of the hot water syabilised. I still think it's too high and above the supposed regulated 55C. The heat store sensor is just inserted into a tube so relies on conduction for a measure. Heat transmitting paste should conduct better than air and I think that should reduce the actual, rather than measured hot water storage temperature.
Rob