Yet another P0420/P0420pd Code Schema

Hiya,

I have a 1997 GMC Jimmy; when I first purchased it, the transmission had to replaced (Issue between shifting between 2nd gear to 3rd gear). That was replaced, shortly after - I blew the motor (Basically, the dashboard interface wasn't exactly well thought out - the 4Low button was right above the CD player button. Hit it accidentally going 70-75 on a highway acceleration ramp, temperature hit 300+ in a matter of seconds. Wasn't fun.)

I let that sit for about 2 years now maybe - and just recently had the motor replaced 2 1/2 months ago. Initially when I got the vehicle back their wasn't a "Service Engine Soon" prompt wasn't on, but within driving it 200 miles (More than 120K on it I'll note) - it kicked on. This was directly after putting gas in it. So, I generally just assumed I didn't turn the cap correctly and thought I could drive it off; that didn't work.

It did flicker occasionally; almost always when I went to bring someone who could check for the codes. But since about Christmas it has not shut off. As you might expect I had to get this inspected - but I haven't been able to. It's my main vehicle and I've already received a ticket to get it inspected, but I can't get it inspected (NY State) until the light comes off; basically the vehicle technician told me not to even waste his time because it was on.

I placed generally all of my expendable money into getting the new motor put in ($600 for the actual motor, $700 for labor). So I'm kinda in a shitty situation here to put it bluntly; if I don't use the vehicle, I can't get my fiancee to doctors appointments (They're looking into a chronic illness, so she's in and out of the hospital regularly each week) as well as I can't take my daughter anywhere, but on the other hand if I do - I get a ticket if any officers spot the lack of inspection sticker when going through toll booths.

It drives wonderfully, drives extremely smooth and handles great even in comparison to modern cars; the only re-occurring problem I'm having with it is the battery keeps dying on me, but that's generally because it sat for so long.

Reviewing the P0420 code, I found a number of possibilities - ranging from an Oxygen sensor (However, I'm confused about this - because some people are extremely hostile on the fact that this code is not a result of an 02 sensor), a leak in the exhaust system, and even a mention of the axle (Don't see why that would be mentioned, but I'm not a vehicle mechanic - so I'm ignorant on the subject), and obviously the catalytic converter.

My question is logically what do you feel could be possible given I have both a new transmission and brand new motor?

(I'll also note a new fuel tank was put in too - however, the sensor is definitely off on that now [Put dry gas in to evaporate the water]).

Logically I'm assuming it's an Oxygen sensor, everyone said that's what it most likely was prior to me even grabbing the code. I can't really afford to get a new catalytic converter, someone said it costs around $1,000? (That costs about the same as replacing the transmission - I was expecting around $150 for that part and between $40-80 for an Oxygen Sensor. Am I wrong?)

The exact codes I got are described below:

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[1] OBD-II Code: P0420

Catalyst Efficiency Below Threshold
(Bank 1)
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[2] OBD-II Code: P0420pd

Catalyst Efficiency Below Threshold
(Bank 1)
===================

On return error 2, the p is over the d. (Does this mean pending?)

98% of the time it's the converter that causes this code. It sets because the converter has lost its ability to store oxygen to reduce emissions. The ONLY way that it could be a O2 sensor is if the upstream O2 is more or less dead and not switching and has the same voltage and switch rate as the downstream, and if this was the case you would have a running issue and another code for the upstream o2 not switching.