A number of pilots have done this and found that the 165A works fine in the COM2/NAV2 position (driving a KI204 or similar) but does not work in the COM1/NAV1 position (driving a KI525 HSI).
On a quick read of the 165A installation manual there is the obvious difference that the 165A contains the NAV/LOC decoder (effectively a KN72) but that isn't used in this case.
The 165A has some config options but again these do not appear to be applicable to the more simple 155A functionality.
Can anyone suggest what the issue might be?
It affects people who believe that their country will require two 8.33 radios - as certain German avionics shops have been claiming, and claiming that the German CAA will demand it; of course this helps revenue generation too 
I have a 165A in the NAV2 position but have not tried it in the NAV1 position since I installed the Sandel SN3500 EHSI. It really ought to work since my KN72 is now gone and the EHSI decodes the 155A's composite signal directly.
However I hear from a pilot with an identical aircraft and with the Aspen EFD-1000 that he has the same problem with the 165A not working in NAV1... would anybody install an EFD-1000 and leave in place the KN72?
Even if it's a slide-in replacement, it will require a Major Change...
With overhauled KX-165As costing $3700, you might be better off getting a Garmin GTN or GNS. The Garmin equivalent to a KX-155A is the new GNC255A which costs only slightly more than an overhauled KX-165A.
I was looking into a KX-165A but I don't think it's a good use of money. Still hoping the German 2 COM requirement will go away.
Even if it's a slide-in replacement, it will require a Major Change...
Only under a brain-dead certification regime 
I think the new GNC255 of Garmin is vary good value for money given the fact it allows ARINC429 output and some other goodies....As retrofit and when doing upgrades in combination with E-HSI efis very nice ..There is no other NAV/COM in that price range offering this......
Forgive my ignorance, but what is an ARINC429 output useful for? I've looked up ARINC429 on Wikipedia, but am little wiser!
ARINC429 is (in this context) a data stream specification, whereby textual data is transmitted serially at a fairly high data rate. The stream contains labels which are standardised (well, not 100% between Avidyne and Garmin in some cases ) and the stuff after each label is the actual data.
So you can have an ARINC429 data stream going to loads of receiving devices at the same time, and each will pick whatever is relevant to it.
In most (all?) cases the receiving device doesn't send anything back i.e. it is not really a "protocol".
Physically, it is a single-transmitter, multiple-receiver system, with differential signalling, like RS422 but AFAIK the signal levels are different. I have done masses of 232, 422, 485 but never done 429 so I have never looked up the details...
ARINC429 is the "modern" way of connecting up avionics.
For example an old directional gyro (KG102A) sends out the heading as X/Y/Z, or it can be done as sin/cos (phase shifted sinewaves), and there is always some error there, so one has "tweaks". In addition, one can wire up the signals in different ways (do a google on "synchro" or "resolver" - absolutely wonderful 19th/early-20th century discoveries) so you can be exactly 90 degrees out, etc. Avionics shops routinely get them wrong and take care of it by tweaking the receiving device. Whereas with ARINC429 there is a Label 320 (also Label 314) which gives you the heading digitally.
It gets better for e.g. pressure altitude where you might have about 10 gray code wires from the encoding altimeter, versus 2 wires using ARINC. Much less wiring and less to go wrong.
I hope I got the above right. NC-Yankee is an expert on this stuff...
As to what the Garmin radios usefully send out over 429, I have no idea. Very little of what they might be driving (CDIs etc) in typical GA aircraft accepts ARINC429. The composite outputs (LOC, GS) are analog but are just 1 shielded wire each anyway. I see they contain the NAV/LOC decoder so if your old HSI is driven from a KN72 you can ditch that, but the same goes for a KX165A.
I know they need GPS input, bizzarely, to work out the nearest radio frequency to offer you. And of course there is then yet another database you need to update, in theory... I read an interesting debate in the USA where the Garmin rep was amusingly challenged as to whether that GPS data needs to come from a certified IFR GPS installation
There is a variety of converters between ARINC429 and other data streams - example.
There are also ARINC429 concentrators which accept a load of ARINC429 (or even RS232) streams and combine them all together, so you end up with just one wire pair which is better. A GTX330 does that, for example, though obviously that is not its primary function
Like most things in aviation, ARINC429 is absolutely ancient and probably goes back to the Apollo program.
Thanks Peter for that explanation.
2280 channels divided by 27 countries = 84 aviation channels per EU country, assuming 2000 mile radio range means that Portugal cannot share frequencies with Romania.
That ought to be enough, and certainly justifies all aircraft owners spending their money on a new radio... or two.
Not all KX165A have 8,33 I understand only the ones with - 0201...I only hope they will not go that far that all two radio's on board need to have 8.33 Khz ...first FM-immune, then Mode S then 8.33 then ADS-B and airline traffic is going down in Europe go figure...
To add what Peter wrote ARINC429 signal levels are floating and transmit in simplex mode (no acknowledgment) and the standard wiring is twisted shielded wire (to reduce electromagnetic interference) with A/B containing the floating signal. Two speeds 12Kb/Sec and 100 Kb/sec. 32 Bit with parity check If I remember well max 20 recievers. Indeed the label identifies the parameter and then you also have equipment identifier NAV1, NAV 2 etc. Some bit are used for TO, FROM etc... Most of the LRU's line replaceable units have a BITE build in test to check if the unit is working properly. ARINC 429 has allowed to set op a central Maintenance Computer from the cockpit to check equipment remotely running certain test routines as we have a data link now...
De signal levels are rather hi and floating thus allowing to bridge rather big distances as you have in airline jet's
ARINC 429 is not going to go away soon as neither XYZ synchro signals. Seeing ARINC429 moving to GA more and more is for me a big plus it's maybe not that fast but it is an industry standard which makes interfacing much more easy as the protocol is known.... And yes the Garmin 330, 328 are data concentrators and can be used intelligently for some air data and ARINC 429 re-routing.
..okay so I had a GNC255 installed in my NAV/COM2 position to replace a 14V KX155 with glideslope and I like it…the audio/tx is clear and crisp and it offers some nice features….and it was the least expensive solution..
I will report on it when having more time behind it..
I will see what happens on the next year to replace the NAV/COM1 KX165 and KLN90B ..not in a hurry now…I can still fly PBN according ICAO Amendemend 38 proposal with RNAV5 through European airspace
