Transmission carnage, O2 sensors, catalyst, and P0138 - next steps?

Finally got my sons SX4 back together after the trans let go, and now need to sort out a lingering emissions problem. 

There is a persistent P0138 code. I can clear it and it returns during the next drive cycle. We've replaced both O2 sensors (the downstream twice), found no exhaust or vacuum leaks, cleaned the MAF and replaced injector o-rings (while we were in there for valve cover gasket).

Currently the sensor is mounted about 12" behind the car using an angled extension to avoid contact with the oil pan. The leads are too short on all the sensors I've looked at to use the bung right after the cat. I'm considering extending the wires to see if moving it closer will help.

Watching the live data tonight the upstream was bouncing around +/- 0.1 mA, and the downstream would stay around 0.8-0.95 V.

If you blipped the throttle the engine the downstream would drop to 0.1 and then climb back up to 0.9 V. 

If you raised the rpm to 2500-3000 and held it steady the downstream readings would drop and then climb, sometimes as high as 1.1-1.2V. This matches what I observed a few days ago while driving down the road at that engine speed.

Tonight we watched the readings while forcing the mixture rich with a propane torch. When adding propane the upstream would show -.2 to -.4 (depending on the propane flow) for a brief period and then compensate to get back to around 0. The downstream would stay above 0.9, and after shutting off propane it would stay between 0.8-0.9V for 5-10 second before dropping down to 0.2 and then recovering to 0.8V. If I supplied propane long enough, the downstream would eventually go over 1V. The more times we tried the propane, the longer the downstream readings would hold in the 0.7-0.9V range.

Im not sure what all that tells me, but I know we have some members with deep knowledge of emissions, so I'm hoping someone can point me towards the next steps.

Im thinking it's a cheap aftermarket car causing the problem, but want to avoid throwing parts at it any more than I already have.

Oh yeah, transmission carnage photos:

You have a floating ground issue.  If the front O2 is switching around, the rear O2 should be sitting around .650 or so.  And it should never exceed .950v or so - the cell doesn't MAKE more voltage than that.

I had two different Nissans that ground the sensors through the ecu, and they acted oddly until the rpm and throttle opening got high enough to go to open loop, then they would run normally.  The ground had failed inside the ecu.

I fixed mine by adding a supplemental ground.  However, now I can't remember whether it was the sensor ground, or the heater ground.

Long way of saying, I agree with Pete.

Pete. (l33t FS) said:

You have a floating ground issue.  If the front O2 is switching around, the rear O2 should be sitting around .650 or so.  And it should never exceed .950v or so - the cell doesn't MAKE more voltage than that.

 

 

+1. 
sensors will never go above 1v or below 0v. If it does, there is a wiring issue. 
 

BTW, WRT voltage ranges, rich is high, lean is low. 

Thank you for the feedback and pointing me in a direction that hopefully will avoid additional parts.