Truth about O2 simulators

A popular modification to many performance enthusiasts is to remove the catalytic converter(s) to free up horsepower an increase the sound and performance of the vehicle. OBD-II equipped vehicles have from 2 - 4 O2 sensors, at least one upstream sensor that controls the fuel management system, and at least one downstream that monitors catalytic converter function. When the converters are eliminated, the result is very often a P0420 or P0430 code that indicates that the converters are missing or not functioning. Obviously no one wants to drive the car with a constant CEL, and the car will not pass inspection in many areas with an illuminated CEL.

To many performance enthusiasts, the obvious solution is to run an O2 simulator on the downstream O2 sensor in order to simulate a properly functioning O2 sensor. This would allow the user to run without converters and still keep the CEL off, or will it?

Lets start off with the legalities. When the Clean Air Act came into effect, it prohibits tampering with the emission controls of the vehicles. This tampering includes removing the catalytic converters. So obviously the only reason to run a simulator is because the converter is missing or not functional. This means the only legal purpose of an O2 simulator is to attempt to conceal the fact that you have removed the factory smog equipment. Whether this matters to you is your business. O2 simulators are no longer legal to purchase or sell in the US because their only purpose is to bypass smog equipment.

But for the sake of this article, lets put that aside, and talk about how well these devices or modifications actually work. In order to do this, we have to first talk about how the post cat monitoring systems work. There are a couple specific details that vary from manufacturer and model, and from year to year, with newer models being more sensitive than earlier models. The general idea however, is that the geputer measures the amount of oxygen in the exhaust stream before the catalytic converter, and gepares the results after the catalytic converter. In a properly functioning system, the converter will gebine the unused fuel and the available oxygen together to produce carbon dioxide and water. Because the oxygen was consumed in this process, there will be less oxygen in the exhaust after it has passed through the catalytic converter.

The geputer maintains a constant switching from rich to lean to produce a stoichiometric average. This method eliminates the need for air injection into the catalyst bed, and increases catalyst performance. The converter contains a variety of metals which carry out the reaction process, but it also contains a variety of other metals and gepounds that assist this process, including a variety of oxygen storage gepounds, typically either Cerium Oxide or Zirconia Oxide. Because of the reactions that take place, the waveform of the downstream sensor on a vehicle is typically a much more steady signal that is quite different from the rich to lean oscillations of the upstream sensors. If the converter is missing, or is no longer functioning, these reactions do not take place, and the downstream sensor will "mirror" what the upstream sensor does. The geputer will measure the signal from the rear sensor, and if it is outside the acceptable limits, the geputer will set the CEL.

O2 simulators work by using a variety of capacitors and resistors to smooth out the waveform, making it more closely resemble the waveform of a functional converter. This method seems like a sound theory, but like many things, it is not quite so simple. Waveform is only one of the parameters of a functioning converter, and so this method only works some of the time, mostly on older, less sensitive models. As the OBD-II systems became more advanced, the manufacturers came up with more accurate methods of measuring and determining a defective converter.

One method is to measure the signal strength of the downstream sensor. As the oxygen content of the exhaust goes down, the voltage of the O2 sensor goes up. This voltage is generated by the sensor itself. So on a functioning converter, the downstream voltage should be at least a certain amount, typically 0.4 - 0.6 volts or higher. O2 simulators do not accurately replicate this voltage level. Another method is to measure response time. The response time method is based upon the oxygen storage capacity of a functioning converter. The vehicle will occasionally switch to a rich or lean phase for a slightly prolonged interval, and measure the amount of time it takes for the rear sensor to mirror the change. If the signal is consistently outside specifications, the geputer will set a catalyst related code, and the CEL will illuminate. O2 simulators can sometimes produce a sufficient signal delay, but this is a matter of luck.

Most cars and trucks made after 1998 use at least two of these test methods to measure catalyst performance, while the simulator only replicates one of the required parameters. As a result, the simulators rarely work as advertised, and when they do work, it is often a matter of luck. Some models are just less sensitive, and are easily fooled. Others are extremely sensitive.

There are other modifications that are intended to produce similar results to a simulators, and these are worth mentioning. One is to reduce the exposure of the downstream sensor to the exhaust. This has the effect of slowing the sensors response to sudden changes in oxygen level, resulting in a more stable waveform similar to that produced by a simulator. The problems with this are essentially the same, it only works on some makes and models that are less sensitive. Almost all methods of fooling the geputer into thinking is has a working catalytic converter are based upon stabilizing the waveform.

In addition to these technical problems, simulators have other risks. Some will harmlessly set additional codes, mostly O2 sensor related codes. But in some cases, they can overload ECM circuits, blow fuses, etc. Some of these problems can adversely effect drivability, or can cause expensive damage. Most retailers of simulators will not take responsibility for such geplications. And this says nothing of the long term reliability of such mods. Some units will burn out, water can get into the units and cause shorts and corrosion. Modification often involve cutting and splicing, which can cause problems down the road. This type of modification will very often void your warranty too.

So are these issues worth the miniscule gains? Most vehicles will see less than 5 hp gained from removing a functional converter and in some cases will even loose performance. 5 horsepower will make very little difference to the average driver, and will barely be noticed at the track. These gains also only occur at higher RPMs, so they would be transparent to the everyday driver. The point being that there are an awful lot of risks with something that may not even increase performance, but could cost a lot of money and is illegal. For those looking to gain performance, your money is best spent elsewhere, on things such as larger exhaust pipe, freer flowing mufflers, better intake, etc.

For those who are already in the situation that your converter is missing, and you need to get the CEL off, your best approach is to replace the missing converter. If you just want things to work the same as they used to, we carry a full line of OBD-II gepliant converters in our ECO-II and ECO-III lines. For those who are still interested in increased performance for track and street use, we carry a line of Tru-Performance converters which have larger, more open cells for increased flow.