I have a 2015 409 with a 16K BTU in front and a 10K BTU for aft cabins. I have not run them much but the aft AC unit has ok air flow and there is no error codes on control panel but it is not cold and did not lower the temp at all. I opened up the back panel at the foot of the left facing back cabin and everything looked new and nothing was hot. Not sure what to do next as the front AC works well. I was going to look for the filters and coils to see if they are dirty, any idea where they are at? Oh yes, I do have good raw water flow for both units and it does kick off when the right temp was hit when I just had the front AC unit on with the two aft cabin doors closed.
Perhaps you have had a gas leak. While putting refrigerant into a marine fridge is tricky because the gas volume is so small, doing the same for a cruisair or similar air-conditioning system is a lot easier and you might think about getting a can of refrigerant and a pressure monitoring system. If not, get an A/C mechanic to do it for you.
If the A/C worked for 5 years and just now stopped working then it is a very slow leak; in my case one of the units stopped working after a year so I needed a technician to find and seal the leak.
There are filter screens on the intake side of the unit. There should be a coarse wire mesh screen on the return vent (probably on the wooden access panel), but it is not the main filter. There is a finer mesh screen on the intake side of the a/c unit that slides up for removal. If clogged, it can hinder the air flow.
Incidentally, 2 years ago one of my units ceased to cool. Technician came out, used sniffer to determine unit had a refrigerant leak. He filled it back up and it has worked ever since. The assumption is that the fill valve had leaked and that just exercising the valve by refilling caused it to seal properly. Not sure if that’s true, but the unit hasn’t leaked since.
As stated by others clogged air and water filters are the main cause of failure and are easy to check.
Are you still getting the correct flow of cooling water through the system? A minimum litres per minute should be listed in the manual. put a bucket under the outflow if you can to get an approximation. Have you recently hauled out and could the water system be air locked? Try opening the bleed tap.
Checking the level of coolant gas is best left to an expert.
Our system died over several seasons and every aspect above checked okay. In the end it had to be sent back to the distributors who returned it fully working within a couple of weeks, but we never got a full report as to what went wrong.
Let's not jump to conclusions and assume a refrigerant problem right away. First you need to confirm that the compressor is actually starting. If the compressor is running and the larger pipe on the compressor is not getting cool then you can start contemplating the refrigerant charge, but first things first!