I recently got a test pipe and as soon as I turned my car on the saw that wonderful check engine light come on. Pissed me off for sure but I spent the last two weeks reading up on the subject (on po.com and other websites) and I came across the concept of hacking into the O2 sensor wires and inserting a resistor to imitate the voltage that the O2 sensor senses when the car correctly makes "clean" exhaust fumes.
I know that there are other ways to trick the ECU but I didn't feel like spending $30 on a stupid O2 sensor imitator or whatever those devices are that trick the ECU. So I took my chances and went to radio shack and bought me the supplies and did the hack. I'm pleased to say that this "myth" actually solved my problem and the check engine light is no more in my car.
So as a tribute to all of you who are scared to do it, heres my super quick mini writeup with diagrams only on how to trick your O2 sensor in believing that your exhaust is nice and clean.
Materials:
1) wire cutters
2) wire strippers
3) 1 10 Mega Ohm resistor rated at 1/4 watt or more. - Radioshack P/N 271-1365
4) 2 Butt Connectors - Radioshack P/N 64-3108
5) Clear or Gray Heatshrink that is 1/4x6" or larger, but I found this to be the best size. - Radioshack P/N 278-1610
6) A heatgun or lighter or anything that will make the heatshrink actually shrink.
7) electrical or duct tape/zip ties.
Procedure:
1) get under the car and disconnect the REAR O2 sensor from the socket and from the cat. To disconnect it from the cat is a bit tricky and a special tool is needed. You can get it done at a muffler shop for little to nothing. Instead of trying to think up a good excuse simply tell them that your getting a new cat and that you need to take the O2 sensor out to put it in the new one or you can tell them that the sensor went bad and your replacing it. Anyway, if you don't have the tools to get it off the cat then get some place to do it.
2) assuming that the sensor is off the car and out of the cat your next goal is to expose some of the gray wire, i believe that there are either three or four wiress, a black, 2 whites, and a gray, I THINK. Regardless there is still a gray wire.
3) cut the wire and strip about 1/4-1/2 inch on each side of the gray wire. This gap is where your resistor will go.
4) take out 1 10 MegaOhm resistor and simply insert one end into the butt connector. Then take the gray wire that you just cut (Doesn't matter which side) and insert that into the butt connector and crimp both sides. Now give a light tug to ensure that both the resistor and the wire are both secure.
5) Now slide the heat shrink over that piece and expose the other end of the resistor, the one that hasnt been connected.
6) repeat step 4 except do it with the other side of the resistor and the other side of the wire. Now crimp. There is no wrong way to put in a resister so don't stress out about putting it in in reverse, cause it can be connected both ways.
7) slide the heatshrink so that it covers the whole area that has been worked on and so that it overlaps the factory black wire
hose.
8) Now take your heating device and slowly heat the heatshrink so that it shrinks over the resistor and the two butt connectors. This makes sure that no metal or connection of any kind will losen or make contact with the frame or any other part of the car.
9) Take the electrical tape or duct tape and wrap it around the whole O2 sensor cable, or atleast the part the has the heatshrink on it. I wrapped electrical tape around the whole O2 sensor cable.
You should now be done and ready to role! Don't forget to plug the O2 sensor back in the car and you have to disconnect the battery from the car for a second from the car so that the computer will reset. Then just zip tie or duct tape the sensor and the wires to the frame and should be good to go!
Below is a diagram of what a butt connector looks like:
Butt Connector
$1.69
Catalog #: 64-3108
Below is what the resistors look like:
10M ohm 1/4W 5% Carbon Film Resistor pk/5
$0.99 Brand: RadioShack
Catalog #: 271-1365 Model: 271-1365
Below is a diagram of what the heatshrink looks like: The one that I used was the largest clear sized one that came in this pack:
This final diagram is a view of what I just explained should look like:
Don't forget to put tape over this whole deal.
Good luck to everyone, I hope that it is as much of a success for you as it was for me. If you have any questions, comments, or problems let me know.
Hack on my lude brothers!
This is a great writeup. Props to you. So does that mean that if they plugged to run a diagnostic, would it detect the hack or just believe that the O2 sensor is working fine?
--Sam
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I bought another O2 sensor wire and when inspection time comes around i'll put the cat back on and use the unhacked O2 sensor wire to pass inspections.
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First of all, you don't need an exhaust shop or a special tool to unscrew the O2 sensor. I have swapped my cat/testpipe on 3 occasions and each time I have used an adjustable wrench without any effort required.
Secondly, there are a lot of people on here who will take your writeup word for word (just look at the NTPOG OBDII workaround – most people don't realize that cutting the wire will produce the same effect as wiring in an on/off switch). SO my recommendation is to just simiply tell people to splice in the connector however they see fit. I personally will not use a butt splice, I believe soldering is the best method but to each his own.
How did you determine the correct resistor? It's an extremely simple solution to a tremendously complicated problem, and you did it just by doing your homework – mad props dude!
I don't mean to criticise your post, I am insanely happy that you figured it out! I'm just saying that this mod will no **** take a normal person 5 minutes to complete, but you're writeup includes all these extra instructions that make it seem complicated.
For those of you with mechanical and electrical knowledge who know how to do this sort of stuff:
Parts:
1 10 Mega Ohm resistor rated at 1/4 watt or more. - Radioshack P/N 271-1365
Procedure:
1) Get under the car and disconnect the REAR O2 sensor from the socket and from the cat.
2) Splice the10 MegaOhm resistor into the grey wire.
3) Reconnect O2 sensor
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Last edited by NotoriouSH; 01-25-2004 at 12:12 PM.
^^^ I just wanted to say that it's a great writeup for those who have NO mechanical/electrical know-how. It may be a little over zealous in writing, but it's incredibly useful for those who want to know every little detail and I commend him for doing so.
i based my writeup on the fact that even though i an mechanically and electronically knowledgeable i still read every detail so i saw it fit to include everything i could possibly think of. In terms of connecting the wires to the resistor, i assumed that not many people have a soldering gun and that this is in my opinion a solution that will work just as good. So if you want to solder it then feel free but I think that using the butt connectors is a solution that still works just as well and uses less effort. It's your choice, I did the writeup based on what I would do.
This writeup covers all bases and serves the broader PO.com community, but thanks for the cliff-notes NotoriouSH.
It's always hard to decide how much detail is necessary to express the job. Too quick and there are tons of questions that take away from the writeup...too long and people get put off by the seemingly hard task. It's funny how this may have been mentioned in a post before, but until it was re-writen as a writeup, most probably missed it. I know this has been discussed in theory in diffused conversations, but I like this thread. Good job for trying this out and writing about it Topic7
As it was said, how did you decide the proper ohms? I think this would be a good thing to do if your using a vafc also, it wont let the computer see anything to adjust....
Re: Confirming the 10 MegaOhm O2 sensor work around
I know that there are other ways to trick the ECU but I didn't feel like spending $30 on a stupid O2 sensor imitator or whatever those devices are that trick the ECU.
Where can i get one of those O2 sensor imitor or a dummey sensor & would one of them be easier ????
I must say, great idea, and I'm going to do this, but should I worry about air/fuel, I may want to run a 2 bung test pipe, one o2 sensor running with the resistor inline going to the ECU and one to an A/F gauge to monitor the mix? Or does this only affect floral (sp?) carbons or something?
I must say, great idea, and I'm going to do this, but should I worry about air/fuel, I may want to run a 2 bung test pipe, one o2 sensor running with the resistor inline going to the ECU and one to an A/F gauge to monitor the mix? Or does this only affect floral (sp?) carbons or something?
I don't know the whole answer to your question but i've been doing some research and it looks like out O2 sensor works by sending between 0 and 1 volts to the ecu. Below is a graph that shows how the ecu knows how rich or lean it is based on the voltage it receives from the sensor.
as you see the simulate a perfect mix, I think you would need to drop .5 volts to make it ideal, or to get it at 14.7:1.
Quote:
Originally posted by 2000scSH Where can i get one of those O2 sensor imitor or a dummey sensor & would one of them be easier ????
Well, according to ZZ site, the secondary sensor doesnt do much. I wonder if this same thing could be done with the first sensor to keep the ecu from modifying the fuel after it was tuned. Anyone know if this is possible
You can get away with this on the secondary o2, as it pretty much is there to confirm the cat is working properly, and nothing else.
You put this on the primary o2, and it will basically make your car run (super) rich all the time.
right, if you're stock. But what if you have a VAFC or some other type of fuel management system. Wouldn't this allow you to set the O2 Sensor voltage at a constant for the most part and then base the maps completely off of what you set it to be. In other words, no ECU intervention in the fuel maps.
The O2 voltage on the first sensor fluctuates with what it detects in the exhaust stream. By splicing in a resistor, all you are doing is impeding a signal that is already constantly changing. It will do nothing except ruin your car's ability to compensate for rich/lean conditions. With normal driving your ECU already tunes for stochiometric, it's the WOT maps on the ECU that are the problem and those do not use the signal from the O2 sensor anyway (closed loop operation).
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^, yeah, and if you replace it with something to provide the ecu with a constant .5v signal, you'll get a primary o2 sensor CEL. It is expecting fluctuation from the o2 sensor while in closed loop.
I just wonder if you could emulate a proper voltage on the first one so you can tune it youself. Even if the car ran really rich you could just pull some out with afc. I ask this because when i tune my car shortly I need to do something at part throttle to get the car running smooth witht he new setup. I can tell that the ecu has done what it can to get the car to run/idle, but its not enough. I gotta get the car to smoth out some. I just dont want the stupid ecu changing my part throttle settings. I need to figure this stuff out.
Its too late for that now.......I wanna make due with what i have....besides, i dont have enough cash to start that. I guess if i can find one cheap i might. But i wanna find a way to do it without stand alone.
PROBLEM!!! My Check Engine Light came on today. I don't know why but i guess the resistor didn't work or something... Anyone have any ideas or suggestions? Yeah Im mad.
Check the crimp connections are not loose. Soldering is more foolproof. Also pull the thing out and measure the resistence (Ohms). It may have failed under the extreme heat. Your resistor may not be radiating off enough "electrical heat" and this is made worse from ambient temps and exhaust temps. You may just need to alter your design a bit. Maybe not a total failure. Also test under code checking conditions. I think the Helm describes the conditions that need to be met to check emissions and generate a code. Make your bypass resistor assembly and then drive at those RPMs for the time specified. It won't always give you a code under normal driving conditions right at first.
i dunno, i have to check it. I popped out the fuse to reset the car so that I can dyno it properly, I just came back from the dyno and I'm waiting until that check engine light comes back on and i'll see what code it gives.
Take the primary O2 sensor gray wire tap into it.
Connect that to a diode, just so voltage doesn't return down this wire, then to a resistor (prob. less than 100 M), then to the gray wire on the secondary O2 sensor.
My reasoning
I beleive the cat would lean out the exhaust gas thus lowering the voltage in the second O2 sensor compared to the primary. The voltage seen in the first O2 sensor would be dropped at a constant rate then and sent to the ecu as the second O2 sensor voltage.
went to buy a diode yesterday, and the guy at the store told me that most diodes have about a .6 v drop, so i'm thinking this will work. .6v may be too much of a drop, but I'm also not positive on the specifics.
Did the data logging get done? If we knew what voltage the O2 sensor produced with/without a catalytic converter it would be easy to decide on which resistor was needed.
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UPDATE.... hehe, i forgot to tell you guys that it snowed here and i think the reason my check engine light came on was because something got stuck to the O2 sensor. So anyway, i washed the underbody and put some duct tape over the sensor and pulled the fuse to make the check engine light go off and since then I havent had the engine light come back on.... its been about 200-500 miles. I also tuned my VAFC so that might have something to do with yet but as of now I havent had a CEL for at least a few weeks!
Well just to help confirm things, can you let us know how you decided on a 10 megaohm resistor? Without an explanation I will be skeptical as to whether or not this is something that is safe and reliable.
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I just used 10MegaOhm because I know that it drops WAY more then the necessary .5 volts and as long as I have a VAFC then I can add gas manually instead of the ECU doing it so I know I'm Okay. I wouldnt advise doing this unless you have a VAFC that's tuned. If you decide to do this anyway, make sure you drop .5 volts. Off the top of my head I think a 27 Ohm resister would drop about .5 volts which would theoretically make the car have a perfect fuel/air mix ratio assuming that it was running super-rich. In reality you will probably need to drop only like .2 or .3 volts so get like a 15-20 ohm resister and I suppose it would work, however don't take my word for it. i think someone should try it and see if it works before I say that it actually works. This is all off the top of my head with like 10 seconds of calculating, so if anyone has calculated this better then me fee free to interject.
The rear o2 has nothing to do with the car's "self-tuning" ability all it does is check that the cat is working right, so your vafc tune should have no effect on it.
So when the catalytic converter isn't functioning it produces a given current, when it IS functioning it produces a current that is .5 volts lower? How did you figure this out is what I'm asking.
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I just did this with the install of a Testpipe and I still throw code P0420.
WTF Mate!?
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The posted links had no beneficial information whatsover. Also, the graph that shows signal dropoff is for a primary o2 sensor, not the secondary. That means the data is worthless and explains why the resistor mod isn't working for MedinaJ78. The idea is good, but the only way to make it work is with data logging so we can calculate the correct amount of resistance needed to trick the ECU!
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your right, however all i can say is that it worked for me, so I dont know what MedinaJ78 did wrong. If he plugged is rear O2 sensor into the test pipe, that might be reason, cause mine isnt plugged into the test pipe, its "hanging loose" (zip tied around the frame of the car) so it reads the outside air instead of the exhaust.
I looked online a found some stuff:
check it out, i think it may help those of you who want to know the techy stuff.
"In the pre-OBDII days, there was only one O2 sensor in each pipe. With the advent of OBDII, the government said that there has to be a way to know if the cats are functioning properly. The solution was to add an O2 sensor to read the mixture after the cat. To explain how these operate together is a little more complicated.
The first thing to keep in mind is that a vehicle with OBDII is constantly self testing itself. Whenever any component or system doesn't show the correct output, the MIL comes on. The computer looks at two things from the O2 sensors - the output voltage, and the frequency at which it switches between lean and rich. The output voltage range is a simple thing to verify. To check the switching frequency the computer uses a pre-determined fuel control procedure that cycles the engine from rich to lean and looks for the corresponding output frequency. If the sensor passes both of these tests, then everything is good and the MIL stays off.
The difference in front to rear O2's is explained by the way the cats work. The front O2's operation is verified exactly as described above. The rear O2's have different operational parameters because of the catalyst. Because of this, the rear O2's will switch at a much slower rate than the front units. As a catalyst deteriorates, the switching frequency of the rear O2's will increase and begin to closely match that of the front O2's. Once that ratio of front to rear switching frequency passes a certain threshold the computer thinks the cat is not effective and trips a DTC.
Your post cat O2 sensor monitors the cats effect on HC/CO and compares it with the pre cat sensors. The pre cat O2 sensors should switch between .175mv when lean crossing to .850mv rich and back again. If the sensor operating correctly its rise time should be less than 100ms and drop back lean in less than 125ms when at operating temp. The post cat O2 sensors (IF the converters are doing the job they are supposed to do), will be hovering (switching) around .400mv to .600mv and back again slowly. "
and one more thing here:
"O2 sensors monitor exhaust gases for the presence of oxygen. Heated sensors are commonly referred to as HO2S, which allows sensors to begin giving valid data much earlier in the warm up cycle. Most sensors output a voltage between 0 and 1.0 volts (usually .10 to .90), 0v - .45v for lean conditions, and .55v - 1.00v for rich conditions. Upstream sensors are primarily used to adjust engine air and fuel flow, while downstream (of the catalytic converter) sensors are used to monitor the effect of the catalytic converter. Sensors do not usually run at constant voltages, rather oscillate rapidly (at least once every few seconds) in a nice sine wave. The frequency is referred to as switching frequency, or as switches. The rear sensor should display a much slower switching frequency than the front sensor, the difference indicating the catalyst capability. These sensors are invaluable in the diagnostic effort, and can indicate conditions ranging from fuel delivery issues, vacuum leaks, to engine oil overfill conditions. "
I know that this is just the tip of the iceberg but I hope it helps some.
The resistor would knock a constant voltage down to the right level, but your post states that the reading is constantly oscillating. I'm not so sure if the resistor is the way to go. Obviously yours must be working because with the o2 sensor hanging loose it would surely trigger a CEL othewise. That doesn't seem like a good solution though, o2 sensors are really expensive to replace and you may want it to function properly if you ever run a catalytic converter again. Can you run the resistor mod with the secondary o2 sensor still installed normally?
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Sharkcohen: I am not sure how to interprete your data. When the catalytic converter is cold, none of that data will be of any use. When did it reach operating temp and what was the average difference in voltage between the two sensors at that point?
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Really, that's surprising that the voltage would swing that much. I thought the end of the second graph (where it smooths out) was operating temp. Is it possible to take all your Excel data and average it out? I don't know how to use the program. If we could compare the two signal's average voltage maybe we would see a trend (like exactly a .5 volt difference).
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good stuff.
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