Update; I found the problem, it is the drain line. Now to get the drain line lose from the faucet, so I can flush it out. Why don't they mention that in trouble shooting??

Sounds like you have an air gap faucet and I hope so.

That PE tubing can get stubborn over time.

Some times careful warming with a hair dryer will soften it up enough to slide off.

Some times one has to cut it short at the faucet nipple with a razor blade, clean it and push it back on... if the installer left enough length and you can reach up under the sink to do it. Otherwise, if there in enough tubing length you can loosen and remove the hardware holding the faucet and lift it up free of the sink to work on the tubing.

If the drain line is repeatedly clogging then you are treating the symptom but not curing the disease.

Assuming this may be an air-gap faucet......In the past, I've had an easy time of it sealing up a little tube to the air-gap hole and blowing into it. Has always freed the obstruction for me and set things right....until the next time.

In my case, the clogging was always at the point the drain tube entered the gravity drain. I finally became annoyed enough to disassemble the drain and examine the entry-point of the tubing into it. I found it protruding insided and angled so that in encouraged rather than discouraged clogging due to output of disposal. Changing the configuration resulted in much less frequent clogging.

The whole thing turned out to be an test of patience. A good friend came over to give me his expertise and help. We took the "guts" of the faucet apart and back together, many, many times. We cut the hoses so they would have a better, new fit, we tried new O rings on the little thing that the hoses attach to, we tried a washer under the screw that holds it all in place, hoping to get a tighter fit, but it continued to leak each time we turned the water on. After doing all of this for over 2 hours, (may have been closer to 3), we gave up. There must be a hidden crack somewhere or something. The faucet is under a 3 year warranty. I'll call them tomorrow and see what they have to say. I've given this thing far more time than I cared to. And I still can't turn on the cold water in my kitchen. Grrrr....

Whoever installed the RO should have installed a shut-off on the water line supplying the RO.

Now would be a good time to install one and you'll have water in the kitchen.

These work really well and cost little...

If the water line feeding the RO is attached with a saddle valve that is a water leak waiting to happen and that should also be corrected.

Your missing part is a FLOW RESTRICTOR and should be replaced when ever the membrane is replaced. It regulates the water flow to drain, not air. If the old one is still in there you can remove the old one and clean it carefully... it clogs with debris.

The shut-off valve pictured above is exactly that... a shut-off valve. It installs in the tubing that supplies water to the RO so you can sht-off the water going to the RO in case you need to service the RO... as you do now. If the tubing that supplies water to the RO is 1/4" PE then you need a 1/4" shut-off valve. If the tubing that supplies water to the RO is 3/8" PE tubing then you need a 3/8" shut-off valve. Home Depot carries them.

Also. mind what I said about a saddle valve. If the line supplying water to the RO is hooked up with a saddle valve REPLACE it with a conventional connection. Saddle valves will leak sooner or later and are a giant water leak waiting to happen.

If any of this confuses you, then get help. This is important.

No saddle valve anywhere that I can see. Will pick up a shut off on my way home from work. New flow restrictor is on the way. Thanks for the help.

I have the flow restrictor, instructions say to insert it into the red tube that then inserts into the membrane. The flow restrictor doesn't fit into the red tube very well, it is a really tight fit. I have pushed as hard as I can and it is only about half way in. Is there some secret way to get it in? Also once it is in how in the world will the water get past it? It seems to me that with such a tight fit, it would act more like a plug. of course it is Sat. and the DuPont customer service line is closed. I really want to get this done today, if I can.
Thanks.

Do-it-yourselfer rule #3... never start a job on the weekend when tech support is closed.

There are different style flow restricters. The ones that I've seen that insert into the tubing have thin "tails" that trail behind them and they need to be inserted in a specific direction.

It's best to speak to DuPont CS... it's their part and their RO and we can't see it over the internet to help you.

I have rule number 3 down now. I'm afraid to ask what rule number 4 is. I have the flow restrictor 9/10 of the way in the tubing. I can't imagine how I will ever get it back out. I bought a shut off valve, not even going to attempt that right now. too many tubes going in all directions. I will try and get a friend to help with that, but he's not available this weekend. Still no cold water in the kitchen. Sigh....

Really shouldn't be that tight a fit. Hope they sent you the correct part.

Are you sure it's in the correct tube and the restrictor is oriented in the correct direction?

You'd have to cut the tubing to remove the flow restrictor if it is that tight a fit.

Rule #4... if it don't fit, don't force it