Downstream O2 sensor fault??

[WDP]

First off, thanks for a great site, ideas and suggestions about MAF clean and TPS sensor cal have cleared up a lot of run problems. Now please help me clear the Check Engine Light on for the past 3 years. I have a 96 4.0 XLT with 184K miles, all long haul stuff with min. stop/go. It runs great, gets good mileage and passes the toughset emits in Pa. I had the faults read, P0136 bank 1 or P0156 bank 2 downstream O2 sensor fault can't remember, but it only has one downstream O2 sensor. Replaced that with a Bosch 15718, checked the wires and connector, reset the computer and light came back on after 2 cold start cycles. The orginal looked OK, no burn marks or heavy carbon. Considering the emits tests, I doubt the converter is shot, what else can I check to rid the car of the CEL demon?? Thanks in advance.

WDP

 

[Jason_25]

Look closer. Your 96 should have 2 upstream and 2 downstream 02's.

 

[WDP]

Thanks Jason, it has both exhaust banks merged into one pipe then a single converter. The sensor I located is just after the converter. I may have missed the other working without a lift, flat on my back in the driveway. Where did they hide the other one?

WDP

 

[Sean90GT]

If you have the 4.0 then you just have the three sensors. Upstream Bank 1 and Bank 2 and Downstream Bank 1.

If you changed the downstream sensor and it did not cure it then it could also be the cat (but you say emissions is fine and no loss of power) or the upstream sensor. Basically all the sensors are doing is comparing emissions going in and emissions going out of the cat. If there is a certain difference then you get a fault.

Maybe try getting one upstream sensor and replacing the old ones one at a time to see if that clears the fault. It may be time to replace anyway and you're only out the $80 or so for the one sensor.

I had a similar code that pointed to a bad cat. Turned out it was the downstream sensor. It's readings were leading the computer to believe that the cat was not doing its job.

 

[Glacier991]

For the price of 3 O2 sensors you could buy a scanner and actually watch them perform and know which are good or bad. It's a worthwhile investment.


Happy Exploring

Chris