I have a 98 Protege 1.5L with 208000 miles. The check engine light came on with a P0421 code. From reading other posts I guess it is the pre-cat and not the O2 sensor causing this problem. Also the car seems to be a little lower on power and the fuel mileage has gone down somewhat. Before buying the cat, I just want to be sure that this is the problem. If I continue to drive it for now will any further damage be done?

Thanks for any help.

Mike

 

that's what it sounds like to me. have you ever done the cats on it?

 

No I have never replaced the cats. I also forgot to mention that after clearing the code, it takes several hundred miles before it comes back and sometimes after it is on it will go off by itself.

I did some additional testing based on information in this post: https://www.mazdaforum.com/m_23609/tm.htm

I did this testing after the car was warm.

At 2500 RPM the front sensor was varying up to about 0.0 0.555 volts (tenths of volts)
rear sensor was varying up to about 0.060 volts (hundredths of volts)

When car dropped to idle the front sensor was varying from 0.0 to 0.625 volts
rear sensor was varying from 0.035 to 0.070 volts


After letting the car idle for a few minutes the front sensor was varying from 0.0 to 0.555 volts
rear sensor was varying from 0.035 to 0.040 volts

The rear sensor voltage is extremely low, so based on the above mentioned post:

Is the front cat bad or do I have a sensor problem?


Mike

 

Could be oxygen sensor to..Pre-cat one. I am having same problem but I just replace the Cta. I suspect sensor