Hi all.

Have searched here and internet exhaustively without an answer to my question.

I have done five LS swaps so far and have never had this issue before.

I just completed a swap of a 2003 5.3 into my old 1953 F100.

I converted to a red blue harness and a 2000 gmc sierra, red blue pcm, as I am running drive by cable.

I am using an aftermarket no name Evil-bay harness. The harness uses the later square plug internal ground heated O2's

The issue is that both O2s show a 4.99 and don't move through 1v to 9v, through out the rpm range, reading on HP tuners. 4.99 being the default when something is amiss.

I have tried four sets of used factory O2's and verified they are good by testing with an OHM/volt meter, so do not believe the O2's to be the culprit.

So checking the harness - with a volt meter - I get 12v and ground for the heater, and I get 4.99V for the signal wire.

The fourth wire which should be pcm triggered ground? shows 1.2 volts - on both left and right banks. Wouldn't think that a triggered ground would show any voltage?? Or is this normal when key on but not running?

If it is not the problem - can anyone shed light on the 1.2v on what should be the pcm ground as in - is it normal?

Rear O2's are turned off by HP tuners.

Assuming all voltage and Ohm testing is correct, on both harness and O2 sensor - where is my problem?

I have heard a rumor that one needs to reprogram the early 2000 pcm to take the newer style square plug O2s in order to read correctly? But can not verify this anywhere.

Is there any truth to this rumor? If so where am I reprograming in HP and how?

One other rumor I have heard is that rear O2's do not read the same as do front O2's? Is this a factor, as these O2"s I am using are leftover rear O2 sensors. Visually they are the same from what I can tell.


Anyone have this problem before?

Hopefully I have expressed the issue correctly/

Any thoughts or repairs to share to get the pcm to read the O2's??

thanks for your time and considerations in advance.

 

If I remember correctly... there was a recent post about O2s and he had rears as front sensors and had problems, he swap in the correct sensors and all good
Maybe you can do a search and find his Thread

 

no such luck - but thanks - no one else out there ever have O2 issues?

 

Seems like there maybe a short from a 5V reference source to the o2 sensor circuit. 1st thing I would try is to hook your voltmeter to the where youre getting 4.99v on the 02. Then start unplugging sensors on the engine 1 by 1 untill you see a drop. Do you have a schematic of the harness?

 

Ok - so problem resolved.

The O2 low sensor feeds( grounds) do not always do a good job - especially if you have case grounded O2's on a dbc pcm that used external grounded O2's

So the solution I was given was to make a secondary ground from the wires going to pins 26 and 29

Also seems that many aftermarket harnesses do not include pin 63 ground - this needs added as well.

Rear O2's are a different sensor range than front O2's and many will argue there is no difference - but I can tell you that of the fours sets of factory truck rear O2's that I have and tested - not a one registered over .499 to .2 volts at any time.

New Gm front O2's installed and they ranged properly from .2- .9 volts as they should after the new grounds were installed.

It was suggested that the odd voltages I was seeing at the O2 harness plug was crossover, not unlike signal lights with a bad ground and power transferring and lighting up the adjacent filament, ie, the easiest path of resistance internally, and I may have to agree as once the external grounds were in place - there was no more voltage issues at the harness plug.