How To: Make Your Own Rear O2 Simulator for $5

Before I get into describing how to build your own rear O2 simulator, I just want to say that I did not come up with this mod on my own. You can search Google and find all kinds of write-ups describing this method. However, I didn't see any write-ups here on T4R, and since many of us suffer from the P0420 disease, I figured I'd share it here as well.

I also apologize for the lack of pictures. I didn't think about posting this write-up until everything was put together and installed on my rig. I encourage other people who do this mod to take pictures along the way and post them up.

You will need to have a working rear O2 sensor for this spoof to work. By "working" I mean that all you have to have is an O2 sensor that's putting out some kind of signal. You're O2 sensor can still be "bad" (putting out bogus signals), but as long as it's putting out something, we can modify it and fool the ECU into thinking that the rear O2 is okay, and that the catalytic converter is working like it should be (even if you don't have one). This method is far cheaper than replacing the rear O2, and is MUCH MUCH cheaper than replacing your catalytic converter.

DO NOT DO THIS MOD TO YOUR FRONT OXYGEN SENSOR. ONLY DO THIS TO THE REAR ONE.

Before you get started, you'll need to run to RadioShack and pick up two items:

A 1 Microfarad (1μF) Capacitor - RadioShack part number 272-1055
A 1 Mega ohm (1MΩ) Resistor - RadioShack part number 271-1134

This should cost around $5 or $6.

Step 1: This step isn't required, but it will make things easier. Unbolt the rear oxygen sensor from the exhaust pipe, which is held on by two 12mm nuts. Then disconnect the rear O2 sensor pigtail from the main wiring harness. With the rear O2 sensor completely removed, you can modify it on your work table or where ever you want, instead of cutting and splicing while underneath your rig.

Step 2: Cut the blue and white wires on the oxygen sensor. Leave the two black wires alone, they are part of the heater circuit and we don't want to mess with that. I chose to cut the blue and white wires halfway between the sensor element and the plug connector. Wire in the resistor and capacitor as shown below. I chose to use heat shrink butt connectors, however if you want to use another method such as soldering, crimping, etc. then that's completely up to you. Use whatever method you like. Just make sure it looks like this:



You'll notice that I have the first diagram labeled "incorrect" and the second one labeled "correct". I've seen some diagrams posted on the internet that show the first wiring diagram. Don't do that one. Your truck will read 0.0 volts at the rear O2 and will throw a code P0136 "O2 Sensor Circuit Malfunction". Go with the second diagram.

Step 3: Make sure the resistor and capacitor are protected from the elements somehow. I'd recommend wrapping everything up in electrical tape. Then re-install the O2 sensor back into the exhaust pipe, and hook the pigtail connector back into the wiring harness. Make sure that none of the wires hang too close to the exhaust pipe (you may have to zip tie things out of the way).

That's it. This should help get rid of your P0420 code for good, whether you have a catalytic converter in place or not. My '99 has had a P0420 code since the day I bought it, and it's been like that for over 2 years. It has the California style emissions and 2 catalytic converters, yet it still threw a code for "Catalyst System Efficiency Below Threshold". Every time I'd reset the Check Engine light, it would come back right away with the same code. After building and installing this O2 spoofer, I have yet to have the Check Engine light come back on.

That's pretty cool! I spent the $100 on the URD o2 sim. Wonder what the difference is

$98, is my guess.

Could you show some pictures of it under your 4runner I'm tired of looking at the damn check light for the same code... P0420 Bank1

Brique said:

$98, is my guess.

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Haha, probably. URD has some big description of why only theirs works, but it's probably a load of crap =)

From the site:

Note: Be cautious of O2 sims that replace the rear O2 sensor with a falsely gererated signal.

Toyota started using a wideband type O2 sensor (air/fuel ratio) in the front position for better engine control. These wideband type sensors need to be recalibrated from time to time by the ECU. The ECU uses the rear sensors for this recalibration process. The ECU cannot properly recalibrate the wideband sensor using a falsely generated signal. This process is explained here, US Patent 5706654.

Use only a URD O2 Sensor Simulator on these vehicles to avoid poor performance, drivability issues and poor MPG.

I thought it was only California emissions trucks that had a wideband in front?

A.Wilson013 said:

Haha, probably. URD has some big description of why only theirs works, but it's probably a load of crap =)

From the site:

Note: Be cautious of O2 sims that replace the rear O2 sensor with a falsely gererated signal.

Toyota started using a wideband type O2 sensor (air/fuel ratio) in the front position for better engine control. These wideband type sensors need to be recalibrated from time to time by the ECU. The ECU uses the rear sensors for this recalibration process. The ECU cannot properly recalibrate the wideband sensor using a falsely generated signal. This process is explained here, US Patent 5706654.

Use only a URD O2 Sensor Simulator on these vehicles to avoid poor performance, drivability issues and poor MPG.

Click to expand...

I agree that a true simulator probably wouldn't work, for the reason stated above about calibrating the front wideband. You need to be able to intercept the signal and modify it, instead of a 100% falsely generated signal. I have a feeling that this write-up, and URD's product probably work off of the same principle, with maybe slightly different values for the resistor and capacitor. That and URD's product is packaged all nice and neat, with connectors for a truly plug-n-play device.

KidVermicious said:

I thought it was only California emissions trucks that had a wideband in front?

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Sort of. The '99 and 2000 Cali models had the 2 cats & wideband up front, Federal emissions had the typical narrowband up front and 1 cat. 2001 & 2002 models all came with 2 cats & wideband up front, which is still commonly referred to as "Cali emissions" even though both Federal and Cali were the same in those years.

Funcrazy1 said:

Could you show some pictures of it under your 4runner I'm tired of looking at the damn check light for the same code... P0420 Bank1

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Sure.

Here's a half decent picture I took this morning.



Looking back, I should have cut out a section of the blue wire to account for the added length of the resistor.

Did this to my '01 this weekend. Followed your write up and it worked. Turned off my CEL due to a P0420.

puppetz007 said:

Did this to my '01 this weekend. Followed your write up and it worked. Turned off my CEL due to a P0420.

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Good deal, glad it worked for you.

I am so deleting my cat.

Thank you OP!

So I won't focus on the point that this mod is illegal (that's not say I don't think you should do it, I think it's stupid laws that make this illegal), though people should know that before they do it themselves, but I will just point out that the URD explanation is probably correct to some extent.

Just dropping a cap and resistor in there WILL change the output of the sensor and change the performance tuning based off the O2 sensor reading. I don't know what's in the URD sim, but it's probably more than just a cap and resistor. They very well could have put in some additional electronics to condition the signal in such a way that it passes, but still gives reasonably accurate feedback for performance tuning. I don't know what inside it or what it does, but if it were just a cap and resistor, someone would probably have figured that out by now.

Just food for thought before anyone feels ripped off by buying a URD sim.

Nelsonmd said:

So I won't focus on the point that this mod is illegal (that's not say I don't think you should do it, I think it's stupid laws that make this illegal), though people should know that before they do it themselves, but I will just point out that the URD explanation is probably correct to some extent.

Just dropping a cap and resistor in there WILL change the output of the sensor and change the performance tuning based off the O2 sensor reading. I don't know what's in the URD sim, but it's probably more than just a cap and resistor. They very well could have put in some additional electronics to condition the signal in such a way that it passes, but still gives reasonably accurate feedback for performance tuning. I don't know what inside it or what it does, but if it were just a cap and resistor, someone would probably have figured that out by now.

Just food for thought before anyone feels ripped off by buying a URD sim.

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How is this illegal if you live in an area that doesn't have emissions or if your car passes the sniffer test?

This is most definitely all the URD device does. The O2 sensor oscillates rapidly between 0.9v (rich) and 0.1v (lean). A failing sensor won't oscillate correctly between the required values. I'm guessing the resistor limits the peaks while the capacitor boosts the valleys of the sin wave so essentially "conditioning" the signal. Not a bad profit margin for them.

Pardon my ignorance but does eliminating the cat really make a big difference?

What kind of difference would you notice by installing the simulator and removing the cat?

Troybylt said:

Pardon my ignorance but does eliminating the cat really make a big difference?



What kind of difference would you notice by installing the simulator and removing the cat?

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No idea. The supercharged and turbo guys would probably benefit, but not sure to what extent. Both my cats are still in place, but I still got the P0420 code.

mxer said:

How is this illegal if you live in an area that doesn't have emissions or if your car passes the sniffer test?

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Any modification of the emissions system on any car from original spec is illegal under Federal law--inspection or not. Obviously, your risk of getting caught is near zero-unless you sell it and the next owner gets popped and it comes back on you--also pretty unlikely if you are careful about who you sell to.

mxer said:

How is this illegal if you live in an area that doesn't have emissions or if your car passes the sniffer test?

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I don't know all the state laws, but you are altering the approved emissions system, so you could be breaking laws in some states, or there could be some federal laws applying too, I'm not totally sure. I know in Cali these are illegal for sure. Some of Cali's CARB laws are becoming federal laws, or soon will be, so there may be something on the books everywhere outlawing simulators for on road use.

Most places don't do a sniffer test on vehicle 96 and newer because of the ODB system. If they let you get away with it, then fine, but just know that it is legally dubious unless you KNOW your state allows it. I mean that is why URD has the disclaimer "Sensor Simulators are not CARB Approved, and are never to be used on Pollution Controlled Vehicles or on Public Highways"

mxer said:

This is most definitely all the URD device does. The O2 sensor oscillates rapidly between 0.9v (rich) and 0.1v (lean). A failing sensor won't oscillate correctly between the required values. I'm guessing the resistor limits the peaks while the capacitor boosts the valleys of the sin wave so essentially "conditioning" the signal. Not a bad profit margin for them.

Click to expand...

Uh, I don't think "most definitely" means what you think it means. Saying most definitely kind of suggests that you have definitive proof that this is all a URD simulator is. If that's the case, then please post some pictures and actual explanation, or at least some other explanation. Have you electrically modeled a URD simulator?

I was just trying to play devil's advocate here. I'm sure URD's simulator is more that just the cheapo cap and resistor the OP used, so just don't bash it until you have some better evidence that they are marking it up 20x of cost.

TheDurk said:

Any modification of the emissions system on any car from original spec is illegal under Federal law--inspection or not. Obviously, your risk of getting caught is near zero-unless you sell it and the next owner gets popped and it comes back on you--also pretty unlikely if you are careful about who you sell to.

Click to expand...

Ah, you beat me to it. I figured there was some stupid federal law on the books regarding this.

I think the only way you will get caught is by the state you are in enforcing the federal law, so in states that don't have inspections, I highly doubt you'd get caught. In cali, you probably will from the description of the CARB checks.

As far as deleting the cats goes, that is alot more obvious for even the most basic inspections, and you actually are making the emissions alot worse in that case. I am against costly regulation that requires 96% efficiency and bends you over the table for new platinum cats if you have 95.5% efficiency, but going to no cat at all really does put more bad crap out into the air, and is not a good solution.

I replaced my rear O2 sensor a couple weekends ago, which meant that I no longer have the mod in place. I did it just to see if both my cats were really bad, or if my rear sensor was just worn out. My CEL popped on not long after, pretty much indicating that my cats are not operating at the efficiency level that the ECU would like. My wife threw away the old O2 sensor with capacitor and resistor still attached thinking that it was junk, so I'll be making another trip to Radio Shack to redo the modification.