I keep getting a P0420 code. I changed out the gas cap and changed out the front O2 sensor(95 has one pre and one post cat). I thought maybe the problem was taken care of since the code didn't come back after 100 or so miles of driving but it came back this morning. Am I correct to figure the only thing left to cause this code is the cat? I replaced it about 5 years ago but the spider assembly did go belly up about 4 months ago and I had gas come out of the exhaust along with some heavy black smoke. I guess that could have knocked out the cat early.
Thanks for any help.

 

I'm not so sure on a 95 but my 96 had 3 o2's before the cat and 1 after, I was getting a code for a 02 sensor and ended up replacing all 4 till the CEL finally stayed off. You can manually shut the CEL off with the right OBDII Diagnostic tool but the vehicles computer doesn't reset itself till you have driven 75-100 miles. Therefore, if you shut the CEL light off after fixing the said problem, and yet it wasn't the right o2 sensor than the engine will tell you after 75 miles that the problem is still there. Basically its the computer protecting itself with out doing any serious problems to your vehicle.

I am pretty sure I can be wrong on some of this info so don't punish me for it. I just know about the 4 o2's only because it happened to me last year during inspection time. When mine didnt pass and the CEL kept coming on after driving 75 miles, the inspection shop owner told me why.

 

This is more geared towards the 98+ schematics, but it should help in the diagnosis:
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Being that this is an OBD2 code, you likely have the fully OBD2 setup in your '95 which means you will have 2 O2 sensors. One directly after the 'y' in the y-pipe and one directly after the cat.

 

After looking over that link I have to think my cat is not doing what it is supposed to do. I have had blazer for 13 years and this is the first time I had this code. When the last cat croaked I had no idea until I couldn't pass emissions. Went through everything until I decided to get the cat replaced. It was hollow when they removed it and emissions went way down with a new cat.
Thanks