Central Computer processing unit connections

How does one disconnect the Computer from the catalytic converters and oxygen sensors. My car 1998 Lexus LS400 has more than 250,000 miles and needs to have catalytic converters replaced. This cost of replacement equal 4000.00 which is equal to the total value of the car. My plan is to just disengage the catalytic-C and Oxygen sensors from the computer that controls engine so that the engine light is not affected by the emission system any more. Can you help me?

What you are trying to do is illegal. Federally illegal. Even if your state has no emission inspection, federal law prevails so… still illegal.

Furthermore, eliminating the O2 sensors from the computer will make your car run very poorly. The sensors are used to run the fuel injection system properly.

If the state that the car is registered in allows it, aftermarket cats can be installed for far less than $4000. O2 sensors are cheap. If the quote you got was from a Lexus dealer, get another from a local independent shop. It will likely be much cheaper… unless you live in California. Then you are better off selling the car out of state.


Illegal and pretty much impossible anyway.


Other than fully agreeing with Mustangman, you refer to fixing this so the CEL will not be affected by the emission system anymore. That ain’t gonna happen.

I might ask what state you live in and whether or not emissions testing is in place. Surely your line of thinking is not that this will prevent a CEL from illuminating and allow the car to pass a test?


Rockauto has the cats for about $200 each (there are two), or $500 for CARB compliant ones (only the left one is in stock, unfortunately).

Anyone here know what the technical difference is between a CARB compliant cat and a ‘regular’ cat? I’m guessing there is a longer required warranty, anything else?

Yes. It has a CARB authorization number stamped into it, and apparently in California merely having the check engine light be off, the required number of monitors completed, and the exhaust gases being within the pollutant levels is not sufficient–the inspector will look to see if the cat’s have a CARB number on them, and fail you if they don’t. Of course, here in Arizona, you can install ANY cat which will keep the CEL off, and make the exhaust gases clean enough–which is how it should be.

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Is there anything required of the maker to get the CARB number? A special test?

Did a bit of searching on the web… seems like the CARB compliant cats must be warrantied and registered with the manufacturer with the customer info. That red tape for the CARB certified parts seems to be the only difference I can find.

I think the Cali requirement against aftermarket cats is outdated now. Since there is a downstream sensor, the cat function is continuously monitored. Who cares who makes the thing as long as it is working, oem or not?


Walker sells both federal and CARB compliant aftermarket cats. Guess which one is more expensive?

Agree, @TwinTurbo, if the cat works as intended why pay extra for registration?

It may be outdated, practically speaking, but it’s still in force. I agree, it’s hard to make a case that the requirement is actually improving air quality. Cheaper repairs = more repairs = better air quality.

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It’s probably something along the lines of the idea that some people will game the system if there’s any wiggle room, and part of removing wiggle room is that the replacement part has to be approved.

I disagree with the idea too, but spending the last few months watching people do everything in their power to avoid complying with reasonable anti-pandemic measures suggested or mandated by their state government makes me understand why it’s in place. Some people make it a point of pride to avoid complying with the rules, or to sabotage their compliance with them by technically complying with the letter of the law but not complying with its intent - like the people who “fasten their seatbelt” by buckling it behind them, which leads to unwieldy and seemingly obvious language in seat belt laws that describe exactly how the seat belt must be fastened.

If people would approach laws as adults rather than as wanna-be disbarred lawyers we could make everything a lot simpler.


I have installed a new aftermarket Cat-Converter (400 = cat + install)and now the engine light continues to come on. I took it back to Lexus dealer and he says the new aftermarket Cat-C is cause for my engine light coming on. I spent 400.00 for new C-C and its a installation. Weather the cat-C is working or not, the engine is working very well and I still get 24-26 mi/gal on highway at 70 to 75 mph. Explain that please.

You say the elimination of oxygen sensors will make the engine run poorly.
My engine is performing very well …24 - 26mpg on highway at 70 - 75mph.
There appears to be no abnormal function of the engine fuel injection system and the car starts up very well when cold or hot. There is no rough idle and acceleration of car is very normal. obviously the operation of the motor and the oxygen sensors is not affected by the defective C-C in the system. How is that possible.? Maybe the computer just by-passes the readings obtained from the Oxygen sensors in the system…a very smart computer made by the Japanese.

Are you saying you don’t have oxygen sensors? That will cause the CEL to light up for sure.

No, this procedure is not to pass local emissions testing. Emissions testing
is not performed in my state. I only want the dash engine light to not be on
all the time. The motor, transmission, differential are all in excellent condition
and therefore, I wish to continue driving this car especially on road trips which we like to make frequently. What does CARB mean? How does it differ from regular
C-C? what is its purpose?

CARB is California Air Resources Board.

Get you codes read and tell us what they are (like P0123, for example) to let us know why, exactly, the light is on.

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You asked what would happen if you eliminated the O2 sensors. I take it from your comment that they are still there but the CEL light is on. So they are still there and still working.

In the original post, you did not say that you had installed an aftermarket convertor just asked what would happen if it were removed. The car will run fine with or without the catalytic converters but it will pollute the air we all breathe more if they are missing. And it is illegal to remove them.

I can’t say why you have a check engine light. Lexus says it is because of the aftermarket cats. I can’t comment because I don’t know what code is set.

If you take the car to most any auto parts stores, they will read the error codes the cars computer in the form of P1234. Post here the codes the store reads and we will try and help you eliminate them


I assume the person you talked to at the Lexus dealer was a service advisor. Keep in mind that very, very few of those people have much of a clue about what goes on mechanically. To avoid looking ignorant they often say things that are incorrect or downright bogus. Maybe you were the victim of ignorance here and the problem is not as severe as was let on.

Over the years only 2 of the service advisors I worked with were knowledgeable and deserved to even have a job at all.
A dealer here ran ads for service advisors. “Computer experience a must. No mechanical knowledge necessary”. And these are the people telling you what your car needs…

Agreed with Mustangman about what to do at this point.


Those are my sentiments exactly, ok4450, The Advisor is totally ignorant
about whats going on.
I will get the codes from local auto store #1,
#2 I will post all the codes found here.
#3. “Circuitsmith” suggested I use “spark plug non-fouler” on the Post-cat
oxygen sensor to fix the CEL issue.
How simple the answer seems to be now. This car still has a good life,
Maybe 200,000 mi or more.
My wife and I love touring in this car all over the country except right now with COVID-19 but hopefully this issue will be solved in next 12 mo.