Engine Mechanical problem
2000 Mazda Protege 4 cyl Two Wheel Drive Manual 70000 miles
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This car has had this problem pretty much since I got it ~1yr ago. it dies after it stays warm for a while, running the heater at full blast seems to extend the range a little bit, but it mekes me kind of uncomfortable...

The mass air flow sensor was covered by an extended warranty, and was supposedly failing (per the engine computer's codes), so I had Mazda replace it.

problem persisted

took it to a shop to diagnose, they said the downstream o2 sensor was faulty and the catalytic convertor needed to be replaced, projected cost was nearly $1000 (OMFG WTF??)

I replaced the o2 sensor

problem persisted

removed the catalyst from the catalytic convertor (I may put a cheaper one downstream later on...) catalyst was grey and melted, obviously shot.

car ran a little better, but still the problem persists...reset the computer, engine light stayed off untill the car sputtered to a pathetic death as I barely got it off the road into my driveway today.

Now, I ask would it help to put in a another new downstream o2 sensor, assuming the first one I put in got burned while living behind my shot catalytic convertor? any other areas to check on that could cause this kind of behaviour?

why would a faulty o2 sensor cause a car to die, seems like a flawed design. old cars didn't have these, it should not be essential for the car to run right? I thought o2 sensors existed to limit emissions and to maximize fuel economy. shouldn't the computer default to a less efficint operating mode instead of dieing all the time and leaving in the middle of the road waiting for the stupid thing to cool off so I can use it again?

The O2 sensors are key in adjusting air/fuel ratio for maximum effeciency.

If out of range they will usually default to a rich condition, which will eventually cause the convertor to overheat and melt.
You will have to replace the convertor and O2 sensor, bu you would also want to check the other sensors (scan tool in live data mode) to make sure they are within specs.
Also check fuel pressure, fuel pressure regulator (gasoline in the vacuum line). Make sure you are getting good spark.

The Mass Air Flow sensor was propably what caused the failure of the O2 sensors and convertor to fail, since it would default to a rich mixture if bad, causing the convertor to overheat and excessive backpressure causing the O2 sensor to fail.