P0156 Trouble code and causes

[mikenearmilwaukee]

Hello, Im not that new to this forum. However, I seldom post unless I can not find an answer already posted. Let me take a second to thank everyone to whom Ive been a somewhat wittless voyeur. As you may have guessed by the title of this topic, I have got the CEL on with the only code of P0156. Not to big of deal right? Just replace the o2 sensor and off we go... Well, not so much. I have a 97 J30. Shes fixing to get her emissions test by the end of February. Ive been trying to fix this code for sometime now. I have of course replaced the sensor on the passenger side on the cat. I have checked the wiring that leads out of the bottom of the car. Is that harness slightly above that sensor for the back two sensors? I cant tell. ? I wiggled them around anyway to hopefully correct a poor connection. However, the scan tool says the heaters on the sensors pass. So, only one of two wires could loose, so I doubt thats the problem anyway. At any rate, what else can cause that code to be set? Is it purely for the sensor itself, or could the sensor be saying there is trouble in xyz system, there by setting the code. If so, what are a few of the systems I can check? There is a small leak somewhere in the heat shield area near the cat, you can hear/feel exhaust coming from it. Also, I replaced my spark plugs with universals. Could that somehow effect it? I plan to replace them with the NKG ones that Ive read so much about here. I did it before I knew of this forum. I just have to wait until it gets warmer here. I can only lay on the ground to fix this car as much as I have to - to get it to pass emissions. Will I pass emissions when not all the systems have passed the computers test, ie, erased code, drove for sometime, then went to get the test... the o2 sensor always seems the last to be tested for some reason - no matter the drive routines I take. Ok, now Im starting to babble and have already asked to many questions. Thank you in advance for any ideas you may have, all that is google has turned up a complete zero for any ideas on this code for this, or similar vehicles. You will be doing myself, and all others to follow a great service. Thank you again.

[driverdriver]

Welcome to NICO!!!

You are aware that there are two sensors right? I would also check the catalytic. PO156 code sometimes also indicates a bad cat.

[mikenearmilwaukee]

Hey, thanks for the super guick reply. Yes, I am aware of all four sensors and their locatios. They each have their own indepent code as well. After a few more inspections, I have found... 1. There seems to be a leak in the pipe somewhere under the heat shield between engine and sensor - seems to be letting more air out then in, but could that cause a lean running condition according to the sensor...? 2. The sensor reads a constant voltage of .6 between the black and gray wires. Where as the other sensor reads a constant .1 -both checked at a lukewarm idle. My CEL is still on and assume the ecm is in "closed loop" - does that mean nonfuntioning o2 sensors? Am I testing the wrong wires? BTW the wire harness for the two rear o2s is under the arm rest, the two simple screws by the cup holder area allow the two item bays to be easily removed, and the harness couples inspected. Also, the airbag computer is in there...? - both news to me. I was shocked to see all the crap and hair in there too. YUCK! Ha. Please let me know your thoughts. -> about the sensors, not the crap and hair.

[mikenearmilwaukee]

Just wanted to post a follow up... I have replaced the pipes right off the manifold. The new pipes came from carjunky.com. They were the cheapest by far at 225$ each. I did check the sponsors of this forum first. At any rate, I erased the code, checked the sensors, replaced the one on the pass side that I assumed to be "sensor 2". The code came back. I replaced the pipes, that were a bit "holey". Code erased. Sounds different, but the code is still gone, all systems are in "has run" status and I passed emissions. I hope this helps someone else.

[yodawill2000]

Glad you got it sorted !!

[brandonjustice93J30]

did the $225 ea include the heat shields?

I have a local shop that is making new pipes for both sides for $250 total, but no heat shields.