you need a different tech/dealer who is qualified to diagnose...how old is your AC?...when was your last service?..."coolant looked OK"...really?...

IMO

Yes, I will be calling a different one. First one hooked up gauges and looked ok. First time one of the 3 filters indoors was dirty. I had replaced them 7 weeks ago. But with 100+ temp he said I needed to replace every 4 weeks. AC is 18 years old, and yes, I would guess most of the techs will just want to replace rather than diagnose. I plan to get a new good quality Dayton capacitor and replace that before calling another tech. I do have a hard start on this one, which was needed when the coil was replaced 3 years ago. A faulty compressor is a possibility I would guess.

be careful with capacitors....

Yes, I have replaced them a few times before. I replaced it a couple of days ago with an old spare I had which has been used before. At that time the tech told me it was bad, but I think he was just trying to inflate the bill as it had never failed to start. I had just called him for an annual checkup. I am checking the drain flow right now, which I suppose could be a problem. But I don't think I have an overflow shutoff switch, unless it comes standard with a Carrier coil. There is no sign of water in the closet. It has run fine all evening, night, and now until 2 pm which is the hottest time of day.

I have tested both overflow drain lines which go outside, and they both are fine. The other drain which goes to the bathroom sink is working fine on the other unit, but the failing unit seems to have some blockage. Anything work better than bleach to free the block? I can rent a drain snake from HD for $50, but this drain makes a couple of 90 degree turns right away.

Our A/C started going on and off yesterday, but about 15 minutes each time. The drain line appeared to be ok going outside but the inside part (not sure what it is called) was wet. This started about 4 pm yesterday, 90+ degrees. Poured more vinegar down, no help. Called a/c guy last night (whom we trust), said he would be out today. Limped through last night warmer than usual but ok.

So he came by today, and said the drain was blocked. He used a "A/C Easy Tee" hooked it to our hose, put it in the pvc drain pipe (where we put vinegar) and told us to turn on the water full force. Some water came out, then none, then WOOSH!!

He looked and there was some white gunk, said yep, that was it. It took him just a few minutes to fix it. We're back down to the mid 70's inside.

He said he sold them, cost $60 and bought one from him. I see that Amazon has them for $28 but we were fine buying it from him (plus his service call charge of course).

So you might want to try that.

Thanks. Looks like that requires cutting the PVC and inserting that device. Did your tech say why the water couldn't just go out the outside drain line instead of the inside condensation line? My unit has run for over a day without problem. It is old and the temp is 100+, it just doesn't seem able to keep up with it.

When warm air is coming out of the vents, have you checked the outside condenser? The next time it happens check if the fan is spinning and you hear the compressor operating.

No, the outside has both stopped. Inside has blower fan going. It has run now for 2 days without a problem.

One problem is wife and kid stay upstairs and are always flipping the upstairs themostat down 4 degrees lower than the downstairs unit. Which means upstairs will run forever and never shutoff. So it might be overheating at the compressor and shutting off.

Then it that case this problem is not being caused by a drainage problem in my opinion.

My educated guess is that the condenser may be overheating and a thermal protection circuit is shutting it down. Did you install the correct run capacitor? When was the last time you hosed down the condenser to remove dirt. If the air flow is poor it will overheat faster.

Yes, I had a backup capacitor and put it in. Then same thing happened the 2nd day. So I don't think it is the capacitor. The fins were very dusty/dirty and I did hose them out. Don't remember if it was after the 2nd or 3rd failure, but that could have been a problem. I was reading the newer coils have a switch which turns outside condensor off if water level in pan builds up. But I would think the water would go out the overflow pipe to outside before that happened. And when I look down the pvc pipes, I do not see any water buildup. But they have 90 degree turns, so I can't see too far.