Help repairing a motor drive PFC board

[abyrvalg]

Full story:
We've just moved into a flat equipped with a split heatpump heating/cooling system made by Saunier Duval (an external compressor unit and 3 indoor blowers), model 31-026 MNHW. All 3 indoor units heat very weak (despite a good weather outside, +8 and higher, this system can heat in much colder conditions), showing a blinking F6 error (googling gives nothing meaningful). A local Saunier Duval service technician called by flat owner seemed to know nothing about the error code, he measured something on a running outdoor unit's control boards according to his service manual (can't find it on the net unfortunately) and told that a PFC board has failed (but not fully, still allowing to run the compressor at reduced power), but there is no more replacement since it is not produced anymore, so the only way is to replace an entire outdoor unit
I'm feeling wrong to replace a big piece of machinery just because of some rather simple power shaping board (no programmable parts, everything can be just replaced in the worst case), so what I've found:

The board under question (marked ZB102B9109 FSLD.CO.,LTD 070505xxf) looks ok, no fried/overheated parts. It consists of:
- TI UCC2818 PFC controller
- IXYS IXGH60N60C2 IGBT transistor - tested with DMM for not failing short in any C-E-G combination. It would explode under such high currents otherwise I guess. How to test it for turning on?
- IXYS DSEI 60-06A diode (tested to conduct in one direction)
- Three big 560uF 450V electrolytic caps (two of them have a bit elevated top centers compared to the third one, some 2-3mm higher - started drying out?)
- Big external toroidal inductor
- Two bridge rectifiers (tested ok)
- Big 75 Ohm posistor with a bypass relay
- Small passives (none looking bad)

Measuring PFC DC output voltage on a running unit (a bit hard to do, need to hang out of the window ) gives 353V which looks higher than plain rectified mains, so the switched-mode circuit should be working somehow at least. The only two feedback paths from PFC board to main controller board (which is responsible for that F6 error detection) are high power DC output and a separate rectified/smoothed/scaled down input mains voltage signal marked as Vsense.

What can be wrong there?
Is it ok to power the PFC board on my table w/o load?
What output voltage should be normal?

[abyrvalg]

Upd: deeper googling by "fsld.co.ltd" led me to a similar (same board codes on diagrams) US market unit service manual, which says that F6 error code has nothing with PFC board, but a problem with one of the outdoor unit's temperature sensors (air temperature, refrigerant pipes), will check them tomorrow.

[abyrvalg]

Yes, that's a broken exhaust pipe temperature sensor. PFC is ok, can close the topic.
Hola Saunier Duval Madrid technicians, replace an outdoor unit because of thermistor failure 

[drussell]

Typical...

"We can't be bothered to actually find the problem with your unit and it isn't new, so you should buy a new one!"  They don't care... They make more money that way!

That is why it is sure nice to know enough about various things to at least diagnose the true problem even if you don't have the facilities to actually do a given repair yourself. 

Glad you found the problem and it is something simple to fix!

[abyrvalg]

If someone will get here from search occasionally:
Thermistor types:
Compressor exhaust pipe temperature sensor: 50K B=3950
3 others (outside air, suction, midpoint): 5K B=3470