I have sel code of P0421. What are the odds of the downstream sensor being bad vs a bad catalyst converter? The car runs great and I get 19+ mpg in town driving. It is a 2001 Mazda Protege, 2.0L V4 in California. Thanks.
Oxygen sensors have a design life of 100K miles…Converters usually last considerably longer…Change the sensors first…They are much cheaper…
Thanks. That was my gut feeling also. Some folks say to change both upstream and downstream sensors. Is that really necessary? Again, thanks.
The upstream sensors are controlling your fuel mixture. The downstream sensors are monitoring the efficiency of the converter(s)…Changing the downstream sensors will probably cure your problem but changing them all on a high-mileage vehicle is a good idea… These parts play a critical role in emissions, mileage, performance and engine life…Some are much easier to change than others…Few can be removed without a fight…
So Caddyman? Are you saying to replace all the O2 sensors, even if there’s no O2 sensor code?
Tester
If I replace the downstream sensor and it solves my P0421 problem will I notice any increase in miles per gallon? Thanks.
No. The downstream O2 sensor is there only for monitoring the catalytic converter efficiency. If this O2 sensor detects any amount of oxygen coming out of the catalytic converter, it means the oxygen is no longer reacting with catalyst and it’s efficiency is falling off.
Tester
I bought a downstream 02 sensor and had my mechanic install it yesterday. So far the light has stayed off. I’ll report back in a week and let you know if it stayed off or not.
Your Protege has an I=4 not a V-4.
That worked for my 97 toyota camry 2.0 I4…One year and half and light is still off… The story is that the front Ox was replaced at the dealer and around 20k miles later the light came on. The price to replace it is very high, compared to the Denso Ox that I got ( I wanted to use the factory installed Ox); very glad it worked. Toyota is very peaky about it. Thank God it worked.
Thanks oldtimer 11. I knew that. Just a typo 
Three days and still no CEL.
On the fourth day the CEL came back on. Now I’ll replace the upstream sensor (more expensive but easy for me to get to) and see if that works. If not, back to my mechanic for a pre-cat. 
Last Monday, while waiting for a new upstream sensor, I turned off the CEL. Here it is, 6 days later and still no CEL.
My Mazda passed smog with one “Not Ready”. Happy, happy!!!
Happiness is a passed emissions test…Glad my suggestion worked for you…