Need help 96' accent code p0421
 

I have a problem with my 96' accent.. Heres the history of the car:
The cars motor blew at like 130000 miles on it, it ran good then except it needed a new catbecause it was clogged, So fixed the cat on it and then the motor blows, no big deal. We find a usedmotor completefrom the Another 96 accent with 80,000miles on it. So I put the motor a year later. every thing goes smooth, all wire hook back up, its runs good.
So I take it to smog so I can register it. Well it fails, but pass emissions part flying colors, butfails for having the battery recently disconnected and not having enough memory in the computer from not driving and putting the new motor in. Which I did put a new battery in it. Guy says to put 150mile to get enough memory and codes stored. So I drive the car, put 300miles on it, and take it to another smog place..This time it Fail for the emissions part, but passes on the codes and battery, This guy says it needs a tune up(wire,sparplugs and it will pass)...CRAP. Well2 days later beforeI do the tune up theCHECK Engine comes on.I use code reader and Get code( P0421 OBD-II Trouble Code - Warm Up Catalyst Efficiency Below Threshold (Bank 1). CRAP.So I erase the code and do the tune up new wires and spark plugs also rember it has a new cat on it. Then take it back to the smog Guy, Well it fails, This time for the battery and not enough info in the cars computer same as before, but passes emission part.So he says to put 150 miles on it and resomg it...CRAP...So now it failed 3 times. So i drive it to put 150 on it, well when I get to 80mile the check engine comes on, and I erase it , and drive another 100 and the cgeck engine comes back on...So thats where Im at... I keep getting the Check engine for CODE P0421...I has a new cat,wires,spark plug and pass emissions parts, but fails for the computer not having enough stored memory, Im not sure but possible because I erase the code. I need help on tryingto fix this Code...It has the original O2's still, with like 130,000, there $100 a piece, so I dont want to spend $200 to find out it didnt fix the problem...what should I do to fix the problem. Oh and I just tried sea foam in the fuel tank and put some through the motor. It helped alittle.

RE: Need help 96' accent code p0421
 

You're downstream O2 sensor is not reading.
No matter how many times you drive and reset the CEL, it will still come on until you replace the downstream O2 sensor.

RE: Need help 96' accent code p0421
 

which one is the downstream O2...Is the one on the header(exhaust manifold)or the one right before the cat converter?

RE: Need help 96' accent code p0421
 

Its the sensor after the cat. converter.
The sensor itself reads the flow of exhaust gases through the converter and tells the ecu if theres a lean, rich, or stoich (perfect mixture) condition.

RE: Need help 96' accent code p0421
 

Mine doesnt have O2 behind the cat or heres a drawling of how my exhaust is
Now is it the first Pink O2 or the second Pink O2 on my drawling.Attachment 2043

RE: Need help 96' accent code p0421
 

2nd pink o2 snesor

And for the record, unless its aftermarket, thats an exhaust manifold. ;)

RE: Need help 96' accent code p0421
 

Well bad news Bears, I installed the new O2' sensor and reset the car computer. Then I drove it for 300 miles, and took it to get it smogged, so I can register the car..which was the main goal. Well It passed smog, just barely but passed, so i get it registered...great...But now guess what, I drove about 50 more miles, and the Check Engine light is back on, with the same code. Now What should I try, I need to fix this problem, it also making the car get alittle worse gas millage...any suggestion on how to fix this problem, so far, it has new cat converters, new o2 sensor and whats listed in original post. I need this problem fixed, any advice?