Ok silly question guys, I just purchased some O2 Sims (Still waiting for them to get here) but I have a question on the install. I slithered up under my car today while doing an oil change and was looking at the O2 sensors after the cats, now do you just unplug the harness from the O2 sensors and plug in the sims at that point? If they do plug in at that point how to do hold them up and away from the exhaust?……….Or do they plug in further up the line behind the heat shield somewhere?



Thanks,

Tony2

 

The rear O2s have really long cables, their plugs is actually up in front of the firewall, very near where the front O2 sensors plugs into the harness. Just keep following the cables to the front and you will find the plugs. That is where you will plug in the simulators. You normally have to pull a little plastic pin out the side of the plug before you can unplug them. The rear O2 has a 4 pin square plug and the front O2s have 4 pin flat plugs.

 

Ok I will have to check to see if my front O2 sensors are still there, I have a LPE TT package and the turbos are right there.........so needless
to say not a lot of room to manuever around either! lol


Thanks

Tony2

 

O2 Sims do not work on the post-cat (rear) O2 sensors.

If you have no cats and nowhere to put the post-cat O2 sensors, then you can just leave the sensors connected to the harness and zip tie them under the car somewhere in free air.

Since you have an LPE TT car, better would be to have LPE turn off the codes in your tune for the rear O2's and then you can just pitch them.

The reason O2 sims do not work for post-cat O2's is that they generate a rich/lean "switching" signal attempting to simulate the signal coming from the pre-cat (front) O2's. The rear O2's do not generate this type of signal, and normally read lean if the cats are working (which is what they will do in free air). If the PCM sees the switching signal from the rear O2's (via the sims) then it will think the cats are dead.

Do not eliminate the pre-cat (front) O2's. Those are important and used to control closed loop fuel. The post-cat (rear) O2's are only used to determine how well the cats are working.

 

Say what? You sure can't run them on the front! And if they don't work on the rear why did someone ever make them?


This is an option that will work to eliminate any codes from the rear O2 sensors. But if your in a state where you have to pass OBDII emissions test you will always have one "not ready" due to the rear cats. Normally they allow no more than two "not ready" before you fail. The O2 simulators will keep your OBDII emissions working correctly.



The rear O2 sensors are exactly like the front O2 sensors except they have higher wattage heaters. Rears produce the exact same rich/lean signal as the front, as a matter of fact you can screw the rear sensor out and screw it in the front, hook it to the front wiring harness and it will work fine. The PCM monitors the rear O2 sensors and makes sure they are switching at a rate NOT equal to the front O2 sensor. That is the only purpose of the rear O2 sensor. If a rear O2 sensor is switching at the same rate as the matching front O2 sensor then the PCM knows the cat between the two sensors is not working and will set a code. If the rear O2 sensors are not switching the PCM will know the rear sensors are bad and set a code. This is what the O2 simulators are for, they produce random rich/lean switching signals so what the PCM sees is the rear don't match the front so it assumes the cat is still working and therefore won't set any codes and will pass the OBDII cat emission test.

Now there is some good advice! You really do need working front O2 sensors on these cars, they won't run worth a crap without them. Which brings up the question again if you can't use simulators on the front or the rear what are you supposed to do with them?

 

Hmm,

Ok now I am confused!

I was going on the premises that I could completely remove OR replace my cats with high flow and use O2 sims to allow the computer to think all is well and not get codes and check engine light?

Am I wrong on this? Or can I proceed in doing so with putting these sims on the rear O2 sensor plugs and ensuring I don’t have problems after I replace or remove my cats?


Thanks again,

Tony

P.S. I can’t send the car to LPE as I am in Canada and it would be a big issue for such a small want.

 

You can remove or replace the cats with a rear 02 edit or simms.

 

Agreed the sensors are all the same. Perhaps I am confused on the switching part. So what causes the sensor signals to switch rich/lean in the first place and why? Why would the post-cat O2 sensor switch at rate different than the pre-cat O2 sensor? What determines this switching rate? And does it behave the same under all conditions?

Bob

 

You are correct! Simply follow your rear O2 cables to the front, unplug them and install your simulators, they simply plug on the harness where you unplugged the rear cables from. After that you won't have to edit anything in your PCM and you can do what ever you want with your cats and all will be fine, no codes, no "not ready" on emissions inspections, or anything else to worry about. Hook them up and everything will work fine, cats or no cats everything will work fine. The simulators are not much larger than the plugs, so you shouldn't have too much trouble finding room for them around the turbos.

A lot of people like to edit out the O2 sensors in the PCM instead of buying simulators because if your having the car tuned (or own your own software to tune) it is free, not cost involved if you already tuning anyway. I like the idea of running simulators myself, they do cost a lot of money but they keep the PCM happy and the PCM will pass all of its test because as far as it knows your cat are still in place and working.

 

Thank you for the clarification!



Tony2

 

Ok, first off the narrow band factory O2 sensors switch voltage from high to low when they cross the 14.7:1 AFR mark. When you go from rich to lean and cross the 14.7:1 mark the O2 sensor will switch voltage, no matter how lean you go from that point the narrow band O2 sensors read the exact same voltage. Same thing on the rich side, once they switch to rich the voltage stays the same no matter how rich you get. Obvious there is a range that they can operator in but the point is the only indication they provide is that the mixture is rich or lean, not how rich or how lean.

Wide band O2 sensors like used for tuning have an output voltage that indicates how rich or lean the mixture is. They don't simply toggle they vary an output voltage. So not only do you know your rich, you know exactly how rich you are.

Now, assume we are at a steady state 60MPH cruse on level ground. Engine is warmed up and running in closed loop mode. The PCM will look at the signal from the front O2 sensor to see if the mixture is rich or lean. If lean the PCM will richen up the mixture slightly and check the mix again to see if it is rich or lean. If still lean it will richen a little more and check again. It does this several times per second. At some point after the PCM has richened the mixture the O2 sensor will switch indicating mixture has gone rich. At this point the PCM knows exactly what the output mix is since it just changed states. Now the PCM will start to lean out the mix again until the O2 signal switches back lean. If you log this with a wide band O2 sensor you will see a zig zag line where the AFR swinging back and forth over 14.7 several times per second. The PCM's goal is to toggle from rich to lean and lean to rich as fast as it can. The faster the PCM can make the O2 sensor switch the closer to 14.7 the AFR will be.

Now for the rear O2 sensor. Assume the cat is working correctly and is burning left over hydrocarbons (unburned fuel) then the amount of oxygen in the pipe just before the cat and just after the cat will be slightly different. So say the mixture is lean and the PCM is moving the mixture towards rich. As soon as the mixture hits 14.7 the front O2 sensor will switch but the rear one won't until the mixture get a little higher. So as an example since the cat is burning the hydrocarbons making a slight difference in the amount of oxygen before and after the cat, the rear sensor won't switch until the mixture actually reaches say 14.8 where as the front one already switched at 14.7. So the rear sensor will follow the front but not at the same time. It too will toggle, just at a later time.

Now lets assume you have a bad cat. With a bad cat as soon as the front O2 sensor toggles across rich or lean the rear sensor is going to do the exact same thing, at the exact same time. This is all the PCM is looking for from the rear O2 sensor. If the rear sensor continuously switches at the exact same time as the front sensor the PCM knows the cat is not working and will set a code.

So all the simulators do is feed the PCM a random signal. The PCM watches the rear signal to be sure it toggles so it knows the sensors are working, and it watches to be sure they don't switch at the same time the front ones switch. As long as the rears switch, and they don't switch at the exact same time as the front the PCM is happy. And the simulators keeps the PCM happy by simply sending it radon switches.

Hope all that made since!

 

Thanks for the response. We tried O2sims several times and had no luck - they just set more codes. That was a while ago and I don't remember the details since we gave up on them. Apparently, some people have had luck with them. Maybe there are different/newer types of O2sims that work better?

I also heard that different C5 PCM's (model years) may use different catalyst efficiency (OSC) measurement algorithms and O2 sensor fault tests, which might also be why they did not work for us.

So maybe the correct answer is that O2sims *might* work...

Bob