the wife bought a plymouth neon se 2.0 build date 9/1999 from some shady people without taking me with and what a mistake! this car had been rigged in every possible way. i put everything back to normal except for the biggest problem. first off the engine was changed and i don't know the year car it came from but the wiring harness that they used is missing the wiring and the connector for the downstream o2 sensor. i couldn't find a wiring harness that was not cut to crap but i did find a downstream o2 harness plug. i need to find a pcm wiring pinout so i can wire this o2 sensor for inspection.

thanks gene

 

Other then you want to get rid of the Check Engine light being on and remove the error code from the missing downstream O2 sensor triggering it from coming back on, you don't need it.

The downstream sensor is really a waste of time and money, but having said that, I don't like the CEL being on for any reason as I know it is something I will have to deal with, but more importantly, I don't want to ignore it, as who knows what else may be going wrong and by ignoring it, I end up with a worse problem!

I don't have a wiring diagram on me at work. I will go home, read up on it, and tell you which wires to connect the sensor to. I do know that the downstream wires come out of the same bundle the upstream sensor uses. I had a frayed wire on my upstream sensor that caused the ASR and Fuel pump relays (on same circuit) to have fits whenever it grounded out.

If I were a betting man, since the wire bundle is located just under the air box, if you remove it, you should see where the upstream O2 sensor wires meet the bundle. I bet the wires were cut near there for the downsteam sensor. You may have two bare wires sticking out that will let you know which two to use.

 

BTW, there is some discussion somewhere in the Dodge Neon forum (may be on neon.org forum, I don't remember) on how to solder in a resistor between the two downstream sensor wires that will insure the proper voltage is sent back to the PCM to indicate "all is well" with the downstream sensor, when in fact, there isn't one. You have to solder in a resistor of the correct resistance to insure that the voltage output is easily within the correct operating range of a properly functioning sensor. A "sensor bypass" if you will.

Don't know if that interests you, but some states will pitch a holy you-know-what when they inspect a car and see that - so beware! I haven't done this mod yet, but my downstream sensor hasn't given me any problems yet either.

Remember, the downstream O2 sensor is for pollution controls, not engine performance, and it really is not necessary to the proper functioning of the car. In fact, a performance PCM does not even have settings for an EGR or the downstream O2 sensor because they hinder performance rather then help it. The upstream sensor is used to determine if the car is running lean/rich and really is useful in that it helps govern the air intake/fuel flow parameters used by the PCM.