Computer keeps throwing P0420 code, need advice

[Tuffguy610]

Hey all,
I am having a problem with my car and I need to fix it so I can pass Pennsylvania emissions inspection. The computer keeps throwing the P0420 code (catalyst below threshold efficiency), which basically means the computer thinks the catalytic converter is bad. I read on here that most of the time this code is set it turns out the rear 02 sensor is bad, but how can I be sure when I don't have access to diagnostic equipment other than a code reader? I have a spare front 02 sensor that is still good, can I modify the harness and use it as a rear? Is there a possibility that I can splice a resistor in the circuit of the rear 02 sensor to lower the voltage thus making the computer think the cat is working more efficiently? My car is old with a lot of miles, the trans has 136k on it, and the car doesn't seem as if its worth putting out $700 for a new OEM cat. Do any of you have any tricks for getting this thing to pass state emissions other than buying a new cat, O2 sims, or an edited tune? Any advice is appreciated.

[00Beast]

An O2 Sim is $50, a magnaflow cat is $70 + installation, and a new O2 is $35.

[azu]

a magnaflow cat is $70 + installation, and a new O2 is $35.

Did that to my 95. I find out afterwards cat is OK. I still have OEM cat just in case. You never know when you might need it.

[willwren]

That old cat is worth $70 at my local metal recycler.

[azu]

That old cat is worth $70 at my local metal recycler.

That is one reason I kept it.

[Tuffguy610]

I modded harness on the 02 sensor I had laying around and replaced the downstream sensor. So far no more SES light. I hit the cat with a few wrenches and it didnt rattle, so it appears to still be good. I was told by a mechanic a few years ago that my cat was inevitably toast because it was throwing the code . I wish I had experimented with this situation earlier so I wouldnt have so many headaches with inspections. I just thought I'd let you guys know so its documented that this is a solution to the P0420 code. Thanks again for your help.