Daikin RZQ100 Super Inverter 2007 ~ U4 Fault

[Makeit go Right]

Been reading over a few earlier posts on similar troubles with U4 faults and wondered if I could run this by the clever types on here, to check I have it correct:

New customer:
Daikin RZQ100 Super Inverter 2007
This single-split has been running for 2-3years in a server room with no history of problems reported.

Breaker tripped (no power outage for premises; just the MCB tripping on its own). After breaker was reset (power restored), the controller was flashing red light and U4-fault showing. The FAQ wall-mount is blowing okay etc but no cooling.

U4 Fault = communications problem between Room & Outdoor Unit.
Engineer went to site and found both PCBs dead (no neons lit up). With this and MCB trip, he discounted a comm-cable problem between in+outdoor units.
Incoming mains power to condenser unit =OK. PCB fuses =OK. Mains arrives at/enters the Inverter PCB and goes out to the second PCB (and mains received at second PCB too). That's good, but the low volt side is not powered (neons etc).

Engineer suspects the transformer on Inv PCB, which should take mains power and feed the low voltage circuitry -- might not be allowing the correct power through (possibly).

Looking for what might trip the MCB and cause trouble for the PCBs, the obvious suspects are compressor motor and the two condenser fan motors.
Engineer checked compressor windings: all equal/good and not down to earth = good.
The compressor label was not scorched/burned etc (clean and neat) = good.
Engineer did not check the condenser fans electrically but said they both spun freely. =good-ish (but no cigar). (I guess he did not have his megger with him, just the multimeter.)

OTHER OBSERVATIONS: Condenser coil: Inside, the coil still looks clean, like new. However, there is a 3mm thick (about) carpet layer of dirt covering the intake surface of the condenser coil (outside). Classic stuff. I suspect that the system has not been maintained since it was installed, 2-3 years ago and the dirt has built up in all that time (24/7, too). And this may have something to do with the MCB tripping? (Increased resistance to airflow and fan motors struggling.)

---------------------------------------------

The plan is:
a) Buy two expensive Daikin PCBs (ouch!!) and return to site.
b) First megger the two fan motors to check they are okay (and make sure compressor motor is rechecked with megger). If any of these items are defective, get new one/s and install them before fitting PCBs.
c) Peel off carpet layer of dirt from condenser coil (clean it).
d) Replace the PCBs and start system up with compressor disconnected. See if all is well now.
e) Then connect compressor and get ready with the amp-clamp to watch the amp draw as it starts up. Any sign of trouble (high amps), switch off before new PCBs duff over.

I must say I am a bit worried about that compressor messing things up when we fit the new PCBs, killing them off, so we have to get some new ones again (2xouch!!) and the repeat visit.

1) Does this all sound like the correct sequence of repair actions? Any suggestions for the engineer that he should be doing along the way, before we go out spending several hundred £s on these PCBs {bit like expensive cartridges for printers: 2-3 cartridges = new printer, but at least the printer then runs okay}.

2) When installing the new PCBs, is this the best way to guard against the compressor (if it is duff or slightly duff) killing off our nice new PCBs? Ie, using the amp-clamp to watch amps draw, as it starts up.