My York Diamond 80 is not heating. When I turn it on the fan starts but flame does not attempt to ignite. The diagnostic code flashes 4 times. Sheet says 'Aux. Or Spill Limit switch'
What are these? Where are they? Can they be reset?

Usually no to a reset. You need to find the cause of the problem before you reset the control anyway.

What is a spill limit switch? The question wasn't answered. How can someone fix a problem if they do not know what a spill limiot switch is? And so I ask again... What is a Spill Limit switch on a York Diamond 80 Furnace?

This question is in part answered at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-faq-northerheat-58313.html

It is not something we want to answer because this safety device might be trying to save your life.

Uh thanks I tried loaading that page and my browser said "no" I though my puter thought the page funky. I will try again. Thanks. :)

Tried again and that page is full of pop-ups. My pop-ups are turned off. I turned my pop-ups on and the page won't load. So a spill limit switch is some sort of safety device? Is it a monoxide sensor?

It is a temperature sensing device. If the flue gasses don't escape the way they should it overheats and trips, rather than filling your home with CO

It can also be positioned to prevent flames from rolling out of the heat exchanger and causing an explosion or fire.

uh thanks I tried loaading that page and my browser said "no" I though my puter thought the page funky. I will try again. thx. :)

I can't explain why the pop ups are you using a good up to date virus program?

It is a link to the FAQ's on Helpmeaskdesk's HVAC start page. That sticky note at the very top of the page that says " Sticky: Heating & Cooling Maintenance, Repair & Troubleshooting FAQ from NortherHeat"

Don't know why the 'puter acts so funny either. 'tis an old abused 'puter. My kids play weird online games-webkins etc. Thank you for the clarification on the spill limit switch thing. I guess I should see a pro :)