O2 Sensor Not Switching, Need Help

[FirstGensForever]

OK, truck is a 96 S10 with a 408/T56 swap, using a custom harness made specific for the truck by Current Performance in FL. Tune and PCM are from NP.

The truck is having an issue of the O2 sensors not switching at idle, but above idle and going down the road they do begin to switch. At idle the LTFT is stuck at +25%, and when driving down the road its all over the place, from all fuel trims in the negative to bank one at +25% and bank two at 0 and everything in between. While at NP last week to get the truck re-tuned, and they also fixed a vac leak on the injectors, i swapped out the Denso O2 sensors with brand new A/C Delco sensors from the dealer, which are Denso as well and it didn't change anything, as a matter of fact i feel like its running worse with the A/C Delco O2's than the original Denso's i had in there.

Using a wiring diagram i got from Current Performance i traced all the wires and made sure they were ran to the correct pin on the PCM, and everything checked out, i have continuity between the PCM plug and the O2 plugs, the ground side checked out as well.

With the PCM plugged in and the Key on, i have 4.5-4.9 volts on both purple wires of the O2's, and also 4.9 volts on the tan wires as well. The Pink wire has a full 12 volts on both and the black has good continuity to the ground junction at the back of the cylinder head, and pin 40 on the PCM. That's with no O2 plugged into the harness.

With the O2 Sensor plugged in and tapped into the purple wire i get 4.9 volts on both sides, and also I'm getting 4.9 volts on the tan wire as well, but i think this is wrong, as I'm pretty sure i should only be getting a 0-1 volt return signal on the tan wire. Also i can feel the sensor heating up so i know the heater side of things is working.

I'm thinking i may have a PCM issue since I'm getting 4.5-4.9 volts on both the purple and tan wires, but ive been unable to find this info online so im needing some help to know if this is right or points to a bad PCM.

[rob]

Ever figure this out? Have exact same problem

[vortecyota]

Sounds like you have isolated ground sensors and a harness/PCM set up for case grounded sensors. If so, you have 2 options.
1. Get some case grounded( Delphi )sensors or...
2. Splice into BOTH tan wires and run a wire from that splice to PCM C1 (blue) connector pin 63.

I think either method should fix you right up. Let us know please.

[vortecyota]

Holy crap! I just saw the date on the OP. Lol

Rob, let us know how it works out please.