How does this happen?

it’s a 22re 1990

 

In the upper left corner you can still see where it is melted. Excessive fuel is one cause of cat failure, and conversely too much oxygen will also. It's a "delicate" balance.

 

Yes, but diagnose WHY that cat failed and fix it. You'll just fry another cat without fixing the root problem(s)

Rich fuel mixtures is one of them. Bad upstream O2 Sensor will do it in. Replace with a direct fit Denso.

Misfires will take out a cat, too. Change sparkplugs and wires if that hasn't been done yet. I prefer NGK plugs and Denso wires.

New cap and rotor certainly won't hurt either.

Look for vacuum leaks, bad vacuum valves, hoses, ect.

 

That’s the reason I advise everyone to keep their OEM catalytic converter unless they know for a fact it’s dead. We had customers drive around for 1-2 years with the check engine light on and broken O2 sensors; converter polluted to hell. When they finally got the issue fixed they’d drive for a week or two, clean up the converter and it was good to go for a smog test. Sure the OEM were overpriced, but they were just bomber.

I’ve spoken with Magnaflow’s tech dept. They advised me that if a O2 sensor fails and the system defaults to a rich condition their converter can be taken out in less than a 1/4 mile. The Calif Bureau of Auto Repair has disdain for aftermarket converters for good reason.