Ferrolli Optimax 25s boiler broken?

Hi - don't know if anyone can help?

No heating or hot water for a few days now, awaiting an engineer who went 'to get a part' and has now gone awol.

The display shows the following sequence

dp 00 28 LU 00 27 HU -- 8.8

None of these numbers are codes in the manual.

Ferroli say different things when you ring and manage to get through e.g. it might be 'the circuit board'. The missing engineer said this could cost £200 (and if we get one and it turns out that wasn't the problem then this part is un-returnable and an expensive mistake!)

The boiler is upstairs in a boiler/water tank cupboard and I can't see where the condenser pipe goes, there's no sign of one outside and I don't know if it is frozen or how to go about unfreezing a pipe that can't be seen?

Any ideas would be gratefully received!