Okay, so here is the freeze data for my P0420 on two different occasions.What do you guys think?

 

Bump.

 

Since you have a scanner, can you plot the IAT reading over time, or just monitor the IAT values?

 

I was borrowing the scanner from O'Reilly, so I can't observe the changes in IAT values. I sure wish I could afford that scanner!

 

Any events that may have damaged the converter such as head gasket? Coolant can damage the converter (silicates). Also long period of rich mixture can damage the converter. These need to be corrected before converter replacement.

Tracking the aft O2 sensor voltage w/ a scanner or oscilloscope to confirm that voltage oscillates low to high at about same frequency as primary O2 sensor is only way to be sure converter is dead.

good luck

 

I want to see if there isn't a fuel system condition already existing because although the head gasket has been damaged twice, the P0420 code existed before my wife even got the car. With the fuel trim levels being slightly high (2.3 compared to the specification of 1.0), I was wondering if there might be something wrong with the fuel pressure or something like that.

However, the rear O2 sensor voltage does oscillate pretty rapidly and over a large range of values. How would one know whether it's the converter or the sensor?

 

Update: something really weird happened today. The CEL went out by itself. What gives?

 

LTFT of 2.3% is good and not a reason for P0420.

Exhaust leaks can cause intermittent P0420. Fuel mixture monitor details are not published but most take several drive cycles to set. The monitor is looking for voltage reversals (0.2-0.8v) on the post cat O2 sensor. When some level occurs on several consecutive drive cycles the code is set.

Good Luck

 

So could it just be that the rear O2 sensor is poisoned from repeated exposure to antifreeze and/or engine oil? I just hope I don't have to dump a huge chunk of money on a converter.

 

Not possible to say w/o monitoring the rear O2 sensor voltage signal. A dead O2 sensor would likely set a code for unexpected performance.

good luck