I have a 98 neon dohc automatic. it started throwing codes 21 and 42, so i replaced the upstream o2 sensor, which fixed it... for awhile. light came back on. on my second look, i noticed the harness you plug the upstream sensor into only has three prongs, im not sure if this is normal or not.

the car runs very rich to where you can smell the gas, and there's visible white smoke. it also idles kind of rough, anywhere between 800 to 1500 rpm, fluctuating. Also feels like I have power loss.

The mods on the car performance-wise are as follows: full header back exhaust, air intake, unorthodox pulley.

my next step would be replacing the harness, then if that doesn't do it to replace the downstream followed by the cat itself. If anyone has any other ideas that would help id appreciate it.

 

O2 sensors should have four wires and four prongs. the upstream tends to give this siganl more often

 

if your interested i have a brand new bosch O2 sensor the one by the cat, never used it on my neon cuz i sold it so let me know if your interested thanks

 

I'm pretty sure there are only three prongs, but I'll check after work(im hard at work right now ) Replace the O2, but if that doesn't work, you'll need to trace the wiring back and make sure that the exhaust hasn't melted your wires or some other short for the sensor. If the wire lays on the exhaust and melts to the pipe, it will stick and ground out. Best to jack the thing up and look from underneath with a light. Are you running a cat? When did this problem first occur, did this start happening after you removed the cat?

 

im still running a cat, yea. just started one day out of the blue, replaced the upstream sensor and it's still happening. I thought it was because a prong is missing, but that could be normal like you said. My next step is to check all the wiring. I'm going to check the downstream sensor tomorrow while i have the car jacked up putting my snows on.

 

ok and let me know if its bad as i have 1 brand new for sale just make me a offer

 

Can you get is scanned to get the "P" code?

Here are other possibilities for the code 21:
P0131 Upstream O2S Shorted To Ground
P0132 Upstream O2S Shorted To Voltage
P0133 Upstream O2S Slow Response
P0134 Upstream O2S Stays At Center
P0135 Upstream O2S Heater Failure
P0137 Downstream O2S Shorted To Ground
P0138 Downstream O2S Shorted To Voltage
P0139 Downstream O2 Sensor Slow Response
P0140 Downstream O2S Stays At Center
P0141 Downstream O2S Heater Failure

Here are others for the code 42:
P1282 Fuel Pump Relay Control Circuit
P1388 ASD Relay Control Circuit
P1389 No ASD Relay Output Voltage At PCM
P0460 Fuel Level Sending Unit & Circuit, No Change Over Miles
P0462 Fuel Level Sending Unit Voltage Out Of Limits, Too Low
P0463 Fuel Level Sending Unit Voltage Out Of Limits, Too High

 

I'm going to get the 'p' code tomorrow, my buddy has an obd2 scanner, so ill post them later in the evening tomorrow

 

thought i'd update here. the problem was the pin in the connector. it actually got pushed back into the connector housing. pulled it back out and problems gone.