I have a 98 Protege. Several years ago I had the P0421 code. I replaced the upstream O2 sensor and the code went away. Now about 50000 miles later, it is back. I never replaced the downstream sensor. Should I try that one or the upstream again? I really don't want to spend the money on a cat since I am just trying to nurse this car along for another year but my emission inspection is due. The car runs fine and I have noticed no change in power or fuel milage. I checked the plugs and they all look good, no fouling. Any suggestions would be appreciated.

Thanks,

Michael

 

Micheal, I had the same code come up (catalyst insufficient warm up efficiency) and I have been able to correct the problem at least temporarily by making sure the vacuum hoses on the intake side don't leak and making sure the nipple for the EGR boost sensor hose on the backside of the intake manifold is not clogged up.

I then take off the battery negative cable and step on the brake so the brake light circuit will drain all voltage from the systems.

I hope that works for you too. If you don't have a lot of cold starts you should be o.k.

I am too cheap to replace the downstream O2 sensor, but our state does not require smog testing. As you, I replaced the up stream O2 sensor some time ago and the next step would be replacing the downstream sensor. But they cost a pretty penny.