How to tell case ground from isolated ground O2s

[blindsquirrel]

This has been explained elsewhere but I made some nifty MSPaint-edited diagrams a while back and haven't used them yet, so thought I'd post the full explanation here.

case ground.png isolated ground.png

Looks crazy complicated, right? Well let's strip away everything that's not directly relevant:

case ground harness.png isolated ground harness.png

The only difference between case & isolated is in the harness, everything else (PCM, OS, etc.) is the same. So if you unplug everything and check continuity, a case ground harness will be open circuit between any of the 4 PCM terminals and open circuit between any of the 'A' terminals at the sensors. An isolated ground harness will have continuity between all the PCM terminals and between all the 'A' terminals.

Looking for whether there's a wire at PCM C1 #63 or not is not a clever shortcut - in a case ground harness there's a wire there also, it's just used as a low ref for something else!

[horsepowerguru427]

Good stuff. Ive had to deal with people installing the the wrong harness with o2 that dont plug into that particular harness. Never realized there was a difference in O2 sensors until i had to wire chase and diag no o2 activity. Turned out the customer got a harness that didnt match the o2 sensors he planned to use. I believe he just changed to the correct o2 sensors (square plug, not flat)

[jsf55]

I understand the difference's in the plugs and how the are differently grounded, but was wondering if there's a different part number for a flex fuel O2 sensor compared too a non flex fuel ? I.E. L59 Vs LM7. I bought mine before i even new it was a L59 version

[blindsquirrel]

So, all you want is a list of the hundreds or thousands of different part numbers for all these different years and vehicles, broken down into which numbers are case ground sensors and which are isolated? Sure thing, I'll get right on that. Might take a while though.

[jsf55]

Hell no !! Just wondered if E85 requires a different O2 sensor if it's all done at the PCM ? We don't have E85 in the UK so unable too test what the readings would be on a wideband

[SiriusC1024]

Both narrowband and wideband O2 sensors read with respect to stoichiometric regardless of fuel type.

[jsf55]

Thanks SiriusC1024

[TheMechanic]

Found a good article on them. See if this helps.
GM used CASE grounded o2 sensors and ISOLATED Ground O2 sensors. The CASE Ground type are only found on SOME 99-02 models.

After 2003, all o2 sensors were isolated ground. GM stopped referring to them as such, because all had changed, and there were no longer two types. for 2003+ applications, I would recommend running the o2 sensor designed for the harness, since the PCM now controls the heater circuit, using older sensors in this arrangement could cause issues.

In case grounded O2's. The TAN wire or TAN/White is connected to the BODY of the o2 sensor, where it threads into the exhaust pipe. There has to be a good ground from the o2, to the exhaust pipe, to manifold, to cylinder head. the PCM uses this as the GROUND reference when reading the O2 sensor SIGNAL on the purple wire. Each oxygen sensor has its own tan or tan/white wire going directly to the PCM. (PCM Blue pins 25,26,28,29) US and Mexico built trucks used this type, and the 5.3's Had flat front o2 sensor connectors.

In isolated ground o2's, the TAN wire from all 4 o2 sensors IS NOT connected to body of the sensor, and all 4 wires go into a splice pack in the harness. Then 5 wires come out of the splice and go to the PCM. 4 of the tans go to the SAME place as above in the case ground type. Then an additional TAN wire goes into the PCM (Blue Pin 63) to a Ground Reference provided by the PCM. this way, the PCM is supplying the ground reference to all 4 o2 sensors, which will be much more reliable then case ground type. Canada built vehicles got this type.

Okay, in 2003, all o2's were now isolated ground type. A change was done INSIDE the PCM so that the splice pack, and the 5th TAN wire to the PCM was not needed anymore. Each of the 4 locations in the PCM for the o2 senor tan wire, is now a Low Reference.

You need to first figure out WHICH TYPE of sensor you have. CASE or ISOLATED Ground. Check for continuity from body of the o2 sensor to the tan wire. Keep in mind, most o2 sensors wires are not color coded same as the engine harness, find where the tan wire WOULD go, and check it to the body of the sensor. This should be done with sensor removed from vehicle, and unplugged from harness, to avoid any incorrect readings. If you get continuity you have the case ground type, if NOT then its isolated.

Next, figure out what type of harness you have. inplug the PCM blue connector, and all oxygen sensors. Check for continuity from tan of one sensor connector to tan of another. If you get continuity, then there is a splice pack on the tan wires, and you have a ISOLATED ground harness.

For an ISOLATED ground sensor to work in a CASE ground harness, you will have to tie all the TAN wires in the engine harness o2 connectors together to a common ground point, could be to engine block, would probably work fine.

I'm pretty sure a CASE ground sensor would work fine in a ISOLATED ground harness without any modifications (other then o2 plug of course). In this arrangement, the o2 sensor would be getting a Ground from the body of the sensor, AND the splice pack in the harness.

The triangle connector o2's I've only seen on the FRONT set, on 6.0L/8.1L engines. 01 and 02 model year, the PCM supplies GROUND to the oxygen sensor heater circuit on the FRONT o2 sensors only. I think this is why GM used a different sensor, is to keep them from getting mixed up. The PCM modulates the ground to control heater current. I'm not sure, but I think you'd have problems with the heaters if you wired the heater in this directly to ground/power. I would not try to use a triangle plug sensor for a substitute someplace else. These harnesses also had a mix of CASE ground and ISOLATE ground sensors in the SAME harness. The front o2's were isolate, with the after CAT's still being a CASE ground. Now this was not 100%, you could have a mix of both types, or all one type. very confusing when you get into the 6.0 and 8.1 o2 wiring.

LS1 F-Body all used CASE ground, up to 02. These used the sensors with female pins, black square plastic connector.