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?

 

Last time I got a P0421 it was the front catalytic converter.....there is one in the rear also. Not cheap !!!!!! Dealers get around $800.00 to do it.