I have a 1996 626 with 164K miles. Recently, the check engine light displayed and I read the code: P0421. I replaced the downstream O2 sensor. Drove for about 30 miles until engine was fully up to operating temperature - no light. This morning, my daughter drove the car about 20 miles, cut it off, and when she restarted the engine, the light was on again! Once again, I am reading the same fault code. The car has new plugs, rotor, and distributor cap. I have been using Seafoam in the fuel. Any suggestions? Could it be the upstream sensor?

 

it's old enough, may need a new cat converter. And I wouldn't use additives unless you have a definite need for them.

search online, see if there's a common issue - http://www.obd-codes.com/forums/view...php?f=1&t=3319

 

I changed the upstream sensor last night. It may not have been necessary, but it looked to be in bad shape. The light came back on today after the car was driven about 25-30 miles, stopped (ignition off), and restarted and driven. Once the fault is cleared, it only reappears if the car is driven, stopped, and restarted and driven. Can anyone explain that behavior? I haven't tried driving a much longer distance without stopping, but will do that tomorrow. Thanks.

 

Fault codes can be set on the fly, or depending on certain conditions... outside temp, engine temp, xx miles under what conditions, only after multiple failed tests, fuel level. So depending on the code, depends on the conditions to set it - which makes it seem somewhat random.

I'll start with this info is for the 2007 mazda 3 - out of my repair manual, but something similar is likely for your vehicle to set the code.
Monitoring the middle and rear O2 sensor, looking for input change varying a lot more than the output change determines that the cat system is bad.
ECT (engine control temp): over 70C (158F)
calculated TWC (three way cat converter - sounds dirty lol) temp > 350C (662F)
Engine speed between 1500-3000 (manual) or 1350-3000 (auto)
Load between 19 - 52% (at 2000rpm)
Time with purge system control not operating > 5 sec

If detected for 2 consecutive drive cycles or one cycle if it's stored as a pending code. it turns on the MIL light.
A bad cat, exhaust leak, loose sensor, bad engine controller are the potential causes.
Good luck.

 

With that code being the only code set I would say your in need of a Catalytic converter. Generally if you have a different problem causing this code to appear generally a p0171 then its a relatively cheaper fix. But with only that code showing its most likely the cat converter. If you have a infrared temp gun shoot the temp at the front of the cat and rear of the cat and you should have about 100 degree variance between front and back. Easiest way though is to actually see the inversion readings on a scan tool.

 

Thanks for the information. I am beginning to believe that the cat may be the issue. However, I still can't get all of the way around on one point. I can reset the code and drive the car for hundreds of miles and the MIL light will not come on. Once I stop, and restart the car (hot or cold), the MIL light will come on within a few miles (10 max).