ok well, i haven't had my downstream o2 sensor installed (took it completely out for like 2 months) and my check engine light never came on. so i then installed a high flow cat. and i installed the same o2 sensor back in (but didn't hook it up) and as i was driving i got the check engine light came on and surely enough its code 21. so i then hooked up the o2 sensor to just see what they would do and i reset my PCM and nothing. should i keep it hooked up or can i unplug it?

 

So it the sensor is plugged in, you don't get a CEL...but if it isn't plugged in, you do get a CEL. Is that right? If so, connect it.

 

no. lol. it was like this. 1) upsteam connected, downstream disconnected=no CEL WITHOUT cat.. 2) upstream connected, downstream disconnected= CEL ON. 3) upstream connected, downstream connected=CEL ON. but what i'm wondering is if disconnecting it will hurt my fuel mileage? and i guess, how do i know if its my upsteam O2 sensor going bad?

 

Downstream doesn't affect drivability. Get the "P" code to see if its the upstream O2 sensor going bad. If it is, it can make the car run terrible and affect gas milage.

 

Not exactly correct Das, '95 and prior the downstream does not affect drivability. '96 and newer, the Chrysler Corp. holds a patent on the technology of the downstream sensor affecting fuel curves. I am pretty sure that Gary Howell taught me that tidbit while we were working on a project together. Really informative guy. I will find and post that information.
I have found that sometimes (it happened in my '98) you can clear the codes, but the CEL will still be on until thePCM runs the O2 diagnostic. It took about a week of driving forCEL to go out after replacingbothO2 sensors. I asked a friend of mine, who is the service manager at a localDodge dealership, and hespouted something off about "embedded codes" and it will clear the CEL on its own. He didn't lie, it did.
I'm not really sure what the hell he meant by "embedded codes" or what his babbling was about, but I do remember thinking "is he freaking serious.
We were playing golf at the time and I am sure that it made me put my next ball in the water hazard. (probably not, but I needed an excuse for a bad shot. Actually, I need alotof excuses for18 holes.)

 

I'd be interested to see that info. I understood the downstream O2 sensor only checked the cat's effeciency. Doesn't the AF/x PCM not even look at the downstream O2 sensor?

 

thankfully i have a 95'

 

u can check ur o2 senor with a multimeter if u get between 5 an 7 ohm its good but i dont kno if that is when it is cool or hot i believe it doesnt matter

 

Good question das, If I am not mistaken, I thinkthat the AF/x PCM does not read the downstream O2 sensor, however the MOPAR performance one does.

I know that neither readsfor the EGR. Stock PCM reads it all, of course I am only stating the obvious!

You can test the O2 sensor with a multimeter but you have to supply power to the heater wire, or just heat the sensor probe with a propane torch. I think there is a how to on the .org page.

 

This is correct (between 5 an 7 ohms) but this only tests the heater circuit of the O2 sensor not the sensor output. In order to test the output (the other two wires) you have to measure the voltage output of the sensor while the engine is running it should vary between 0.1 and 0.9 volts.