[B3NN3TT]

I've started getting a CEL. Took it to Autozone and had it read, and the scan revealed an O2 sensor problem - Bank 1 Sensor 2 (which I assume is the lower sensor, behind the precat). The thing is, I have no precats, and have installed MIL Eliminators on the lower sensors to PREVENT said CEL's. I unhooked the battery for a few hours, and got rid of the CEL. But it came back the very next day.

So I guess my question is twofold: first, am I correct in assuming that bank one, sensor two, is actually the lower rear sensor? And second - if so, what causes this particular CEL? Will a bad O2 sensor throw a CEL, even through a MIL Eliminator?

I wish I could remember the code, but I don't...

 

[JohnnyRed]

A bad O2 sensor could potentially throw a CEL. What an O2 sensor actually does is create a voltage to let the ECU know that it is working properly. What the MIL eliminator actually does is modify this voltage to a level which the computer thinks is working properly. If the O2 sensor stops working/goes bad then there is no voltage present, then your resistor and capacitor does nothing. If you would like a cheep check hop down there with your multimeter in hand and check it, then compare to the voltage of your other sensor.

 

[B3NN3TT]

Well, I have a multimeter, but I'm not sure what or how to check. Care to give us a quick how-to?

 

[JohnnyRed]

Assuming they are home made......
1 You will want to crawl under your car and get access to the wires of the o2 sensors. The best bet here is to strip a bit of the Grey and Black wires (black wire is not necessarily needed because it is a reference voltage), do it after your Resistor and Capacitor just to make sure your MIL elim, is working. (so it goes, O2 sensor>Mil eliminator>striped wire for testing>ECU)
2 Start your car
3 Crawl back down and Ground the black contact of your multimeter.....anywhere on the frame/metal should do. NOTE!: The black wire for your O2 sensor is NOT ground, it is a 450 millivolt reference. This is basically just a comparator circuit.
4 Measure the voltage of both the Grey and Black wires. (note the black wire should be the constant of ~450 millivolts)
5 Now Measure the voltage of your other O2 which you know is working
6 Comparison is the easiest/best way to find out if it is a bad sensor but also.....if the reading from the Grey wire is above 600mv then it throws a CEL for a rich condition. The readings on the multimeter will most likely fluctuate from 0 to 1 volts quickly. If it is slow then could be a bad sensor/MIL eliminator.

 

[B3NN3TT]

Hrrrm...mine are not homemade. I have the ones like they sell at spmotorsports.com, these:

Oh, well. I think it's a safe bet the sensor is bad, as the code didn't say anything about the catalyst.

As long as I can get a verification that "bank 1, sensor 2" is the lower rear sensor, I'm just going to replace it.