U4 error on Daikin fan unit model FTK25JVE 9

[Fractals]

Hello to everyone,

Need your help guys on how to fix my a/c.

Outdoor unit – RK35JVE 8
Fan unit - FTK25JVE 9

Symptom: Turn on, green lamp on fan coil blinking and louver does not open. I checked the condenser, it’s not working. Strangely, the a/c works in some days, but most of the time it doesn’t. I called a Daikin technician once, did some troubleshooting and all he said was replace the a/c since it’s already 10 years old. I don’t really believe him and I think he was just lazy to do further investigation.
I’d like to try my luck at first before calling another technician or replacing the unit.

Thank you.

[install monkey]

had the same issue with a 5kw split- i installed 6yr earlier
it states check the comms line (no 3) on the interconnecting cable- check your getting 150v ac pulsed signal- which i was- the conclusion was indoor pcb- 80 quid later, replaced pcb and same issue, tested interconnecting cable ok, turned out to be outdoor pcb, easiest way to check this- is turn the outdoor unit off at the isolator- wait a few mins, then power it up and listen for a faint whistling noise- note the green led on the outdoor sub pcb will flash as the indoor pcb, - so proove your cable is ok, if theres an indoor isolator/spur- check the wire on cable 3 isnt switched, check no rodents have nibbled at your cable, theres no condensate pump wired to 2 and 3 wire( should be 1 and 2)
outdoor pcb was about 140 quid

[frank]

Your indoor and outdoor units are mis-matched