I cut off the catalytic converters on my 2001 Navigator and i can still see daylight through them. How do you know if you need new ones? I also changed all the coils and plugs but still losing lots of power on the slightest incline and check engine light flashes like crazy

It gets chokey and will no longer remove the carbon monoxide, changing into CO2-

How do you know if they are plugged up after you remove them because i already removed mine and i can still see through them

well, 'plugged up' refers to impeding the flow- It's not a muffler, just a chamber for an electrochemical reaction to occur. put it back on and take it to an emissions evaluation machine, like the kind at your smog inspection station-

pretty hard for a person to take in carbon monoxide and turn it into carbon dioxide. That's why we need these things, so you cannot commit suicide with your exhaust as easily.

well the active material has apparently been burned away...perhaps by too rich or too lean a fuel mixture-

I can remember a Camaro came into the shop, was running like hell, and I looked underneath and the catalytic converter was cherry red! Good thing the driver did not park on some leaves-