Help Me Understand Oxygen Sensors

[Clarkbug]

Hey All.

Looking to pick the brain of the gurus that understand engine controls better than I.

Specifically, trying to get a handle on the readings from an oxygen sensor, what they should be, and how that impacts other stuff.

Background: Recently had a coil pack go bad on me, and had to limp home on two cylinders. Predictably, Im now getting the dreaded P0421 code. I have an ultragauge, and just clear it as a pending code. Inspection with emissions is due in a couple of months, so trying to see what I can do to fix it other than replacing the precat.

The ultragauge lets me check fuel trim and also the O2 sensor readings. Front and back start out around the same, but the front seems to bounce around a LOT, and Im curious if perhaps I killed the sensor instead of the cat (140k on the car, sensor is original). Should the front reading be higher or lower than the rear sensor?

Any input here is much appreciated. Would be good to know before I start buying bits and pieces if I can just buy a sensor and be done with it....

Thanks!

 

[pcb]

You almost certainly fried your pre-cat,...

You can do the non-fouler trick and clear the code and pass emissions.

http://www.mazdas247.com/forum/show...fouler-trick-to-fix-P0421-for-noobs-by-a-noob


P.S. Your O2 readings are normal,.. the front O2 sensor reacts faster.

 

[pcb]

Here's the info on the O2 sensors,...

 

[Clarkbug]

Thanks PCB.

I went and pulled those same pages after I posted, completely forgot I had the service manual. (Suppose thats a good thing, the P5 has been runnin' alright....)

I was thinking the voltages were the thing, not the inversions. Just trying to see if it was the possibility of a bad O2 sensor instead of the precat.

Biggest concern is that the 140k of miles and rust will make it a less than fun job to swap out. May have to ebay a precat, or go non-fouler.

Thanks again for the assist.

 

[pcb]

Biggest concern is that the 140k of miles and rust will make it a less than fun job to swap out. May have to ebay a precat, or go non-fouler

Those bolts will be nasty,... you might get lucky and they'll crumble for you. The non-fouler is cheap, fast and easy,...

 

[rit14623]

I usually replace mine after 80,000 miles.