#1. No and no. #2.Not likly because it would unlikly cool first 15 minutes. Yes,any compitent tech will be fine. You mentioned contactor,was that outside on heat pump? Watch outside fan to see if it slow's after a few minutes. Make certain there's nothing blocking outdoor coils (plastic bag,grass ect).

"Watch outside fan to see if it slow's after a few minutes."

Nope in fact the fans cheerfully keeps running during thermal cutoff, both the condenser fan and, quite idiotically, the indoor air handler. Because I had to have ducts in the attic, this means the air heats back up, especially during the day. One overall systemic issue I've decided I need to address is the attic ducts really should be much better insulated. Might look into those insulation-only specialty companies that comes in and sprays some kind of foam mix that sticks to them.

Yes it was the main contactor and I put the correct OEM part in:

HN51KC024

Thanks for answers to #1, it's good to know that any charge low enough to cause a problem should trip the low pressure switch. At least I can rule that out. It still seems that...assuming the compressor isn't just dying (Copeland scroll on this one)...there must be some other component in the refrigerant handling chain that is causing this problem. I did test compressor resistances after it came out of thermal lockout and they seemed about as expected, though I didn't go find the chart from Copeland to get the exact figures. No open circuits (or of course it really would be dead!)

btw I've been meaning to get a clamp amp meter anyhow, so on Wed. I will test the old and new capacitor to see if they are somehow causing different draw from the compressor. it seems more likely to me that the cap. would fail in a way that keeps the compressor from starting at all...also, it doesn't seem to make a straining sound or anything like that indicating a gross electrical problem. Basically the return pipe just stops being cold, and the compressor turns off at that point.

of course at this point I'm happy to have a tech out anyhow, I'm just tired of dealing with the carrier dealer I've dealt with before.

My experience has been that caps either work or they don't if they are the correct rating (up to 15% more capatience and higher voltage is ok). It's really sad some dealers give manufactures a bad repitation. There was a time when Lennox pretty much sold to their dealers only and earned a solid following. When York switched to aluminum coils and other cost saving steps it got so bad they wouldn't warranty a compressor before they examined it back at the factory.

thanks. especially because the temps are supposed to be a bit more sane this week, I think I can wait until, hopefully, a tech gets out here late in the week. It is able to do some cooling. If you mean to bypass the actual overload PTC on the compressor that is cutting out...no I think that would be insanity. It's there for a reason. Or do you mean another thermal switch? My understanding is code 72 relates only to the compressor itself. There are other messages for other sensors. The troubleshooting guide says:

If the control senses the compressor voltage after start--up, and is
then absent for 10 consecutive seconds while cooling demand
exists, it will de--energize the compressor contactor, keep the
outdoor fan operating for 15 minutes (if 230v power present) and
display the appropriate fault code. (See table 3) Possible causes are
compressor internal overload trip or loss of high voltage (230V) to
compressor without loss of control voltage.

Now the interesting thing is, before I replaced the contactor, I was getting a lot of "no 230V to compressor" messages, in addition to the thermal cutoff messages. Now I'm just getting the thermal cutout messages. The control board on the outside unit has two leads that attach across the common and the start coil of the compressor, to detect the voltage. So I think if you are just getting the thermal cutoff message, it somehow detects that 230V is still on the line (contactor working), but there's no current flow in the windings because the internal overload is engaged.

BTW years ago when this unit first started having trouble, I did get 82 codes (back then, the guy said it was the capacitor but I'm not privy to how he tested it) 82 is what happens when you have 4 consecutive 72s. These days I shut it off at the first 72 code and let it cool down. Now that I think about...Occam's razor at work...that could be it all over again. Maybe even though the cap. tests ok, it isn't.

btw thanks again for your help. New cap on Wednesday. We'll see, I'll post a followup.

actually now that I look again at the troubleshooting guide, apparently the way it works is

send 24V to contactor - get no 230V = error code 74 (no 230V at compressor)

have system running normally - 230V suddenly disappears from windings when it shouldn't = error code 72

so the bad contactor was at times keeping it from even thinking the compressor was getting engaged.

Yea I flubbed the thermal thing,I was thinking about low aimbent temperture lock out. You are on task about compressor kicking out on overload,that cam mean serious trouble. That inductive amp meter can supply a wealth of valuable information. Those codes hint you may have trouble with start relay/cap/circuit or as you mentioned,run cap or circuit. It's more involved to test than I feel comfortable helping you with so I'm glad you can wait for help later this week.

The capacitor did not fix it. I still think the only remaining cause is something insidious involving the refrigerant line; TXVs can go bad right? Anyhow a tech is coming out.

Is $700 to replace an indoor TXV reasonable? Evap unit is in an attic with stairs access and no trusses. He said the evap. will have to be pulled out to reach it, and a furnace vent removed. Of course it includes replacing the filter drier.

$700 doesn't sound bad if it's guaranteed to fix the problem. Get an understanding before work begain's.

yeah, exactly, I asked them to email me a formal written estimate with the exact replacement part # they are replacing. But it still seems kind of high unless they have some guarantee. Oh well. You only have so many choices around here...the issue with being in a rural area surrounded by major urban areas. Not only are the original installers no longer Carrier authorized, the next company I started using no longer are either! Just noticed that today on the Carrier website. Maybe because they both stunk.

BTW from reading online it seems that it's not uncommon for Carriers of this vintage to have bad TXVs. I guess they buy one they know will last through the 5 year parts period and no longer LOL.

Well david,I'm not paying contractors to do work they can't assure me will cure my problem. Making the correct diagnosis is #1 top requirment for techs. I can train a fool to install parts but it require's trained professionals to determin which to replace. For me it's fix it for $700 or leave it alone.

The TXV valve fixed it. The price ended up being a little lower.

Well, one person liked your post on this three year old thread.