I opened the door and set a colder filament profile, let's hope it works.

I assumed the Core One would be able to manage the temperature better.

Yeah.. The whole "printing with the door closed" is more of a thing for rooms that won't go above 23°C. Or if the room is hotter, either use the 0.4 HF nozzle but slow down the speeds a bit or use a bigger nozzle. 

Of course we can't always schedule the prints depending on the weather though.. 

The 0.25 nozzle is quite a princess in this regard too, when printing PLA. PETG would probably still be fine.

Here in Northern Germany, I only had 3 days above 24°C room temperature yet, so I could print PLA with the 0.25 nozzle and the door closed. The chamber helps a lot with warping! But at 28°C room temperature, you probably don't have much warping anyway, if you can block wind. 

I should have known better. I always leave the door open on other systems when printing PLA. I assumed the Core One would be able to manage the temperature better. Glad this is an easy fix. 

Seems to be a nozzle clog.  I unload the filament, heat the nozzle to 235 or so, then reload the PLA and it runs fine, at least for an hour or so.

Been there too..

Does anyone use the nozzle tool available on Printables that allows you to change the nozzle without disassembling the wires or using a wrench on the hot end (only on the nozzle)?

I simply don't tighten the nozzle that much. I hold the hotend with my left hand in place and use just a naked socket with my right hand.
I tried to use the nozzle changing tool, but my cable routing (kit) is too tight to use it.

You're not "clogging the nozzle", you're getting heat creep. The filament in the extruder gears gets too soft and can't be pushed through the tiny nozzle.

Solutions:
- Reduce chamber temp. -> open the door or increase chamber fans
- Increase heatbreak fan -> Prusa Slicer in Expert Mode - filament settings - custom G-code: "M142 S36 ; set heatbreak target temp" set this to S30
- Print slower, so the extruder will need less force to push the filament through the nozzle

Frustrated because out-of-the-box the Core 1 was magnificent.  My first few print jobs were flawless - including a relatively big job with PC.  I had to change the nozzle, and now I'm having trouble with PLA.  I don't know if I've ever had PLA jam in a printer before!

Seems to be a nozzle clog.  I unload the filament, heat the nozzle to 235 or so, then reload the PLA and it runs fine, at least for an hour or so.

My next clog will get me reinstalling the nozzle.  Other than thumb screws snug but not too tight and the nozzle in place but not jammed too tight, what other advice does anyone have to offer?  Does anyone use the nozzle tool available on Printables that allows you to change the nozzle without disassembling the wires or using a wrench on the hot end (only on the nozzle)?

i think i found the issue....the two screws which adjust the pressure of the pulley which presses the filament on the "big wheel" where far to tight screwed in (pressure on the filament was far too high, nearly sqeezing it)

@GBMaryland: Perhaps it is your issue, "pressing" your PLA filament too much that it broke...

Rest i checked was okay...

Again thank you

will check these....thank you

And yes set the 0.25mm and unchecked the high flow.

All without any issues, but only with standard and matte PLA & PETG. 

Two things to make sure: insert the nozzle all the way against the end-stop, but don't press it against the end-stop, while tightening the thumbscrews. Only very gently lift the nozzle to barely tough the end. 

Also, don't apply rotational force to the nozzle. For me, the hotend and thermistor cables kinda force it to rotate a bit while inserting the nozzle all the way in. 

And don't tighten the thumbscrews too much. They just need to hold the nozzle in place without being loose.

The issue behind these things, is that you pre-load the loadcell, if the nozzle is strained or pressed against something and calibration/nozzle cleaning/probing can fail. 

Did you pinch the thermistor cable between the print head and the thumbscrews? Happened to me.. So maybe your temperature reading is off? 

And lastly and the most important: did you change the printer menu and the Slicer to the 0.25 mm nozzle size and unchecked "high flow"?

Otherwise the printer will indeed try to push the filament into the small nozzle the same way it does for the standard nozzle.