Catalytic converter?

Over the past few months I have been getting a P0420 code on a 1999 Acura CL 2.3L. It happens maybe every 1000 miles. At first I simply reset the code then changed the speed and O2 sensors. Seems an inexpensive try and an easy DYI. I thought I fixed it because I have driven the car for 3000 miles before the problem started again.

Today I monitored the O2 sensor (only has one) and the values are a little concerning but I am not sure what exactly it means and what to look for next. The monitor read between 0.155 and 0.850V at around 2000rpm and rapidly fluctuating up and down between the values. I heard and read that the value should be more or less stable at 0.5V. I am afraid it might point to a bad catalytic converter but am open to other ideas to look at. Btw, I do not hear any rattling or “funny” noise coming from it while driving or banging on it. I also passed emission testing (reset the code a few days prior).

As a non-mechanic, my experience with watching O2 readings is that they are supposed to fluctuate wildly as they report changes in the O2. When the readings would get lazy though and not change rapidly, thats what triggered time for replacement. I suspect what you have is an intermittent miss that could be caused by plugs, wires or maybe a coil.

You need to have your vehicle inspected by a good independent mechanic. About 50% of the O2 sensors and catalytic converters that are replaced in this country are working perfectly when thrown on the scrap heap. A catalytic converter is easily tested so you want to prove it’s bad before getting it replaced.

Today I monitored the O2 sensor (only has one)

Can you look again? A 1999 vehicle is OBDII, which should have two sensors, both upstream and downstream.

Where was the location of the sensor you looked at? Was it before or after the converter?

JoeMario: yes. you are absolutely correct. There are two sensors. My mistake, I did not scroll down on the display to see the second reading.
The last readings that were taking probably were not too accurate because the engine was cold. This morning I took it on a 1 hour highway drive with life monitoring, going mostly around 60mph…

O1 Sensor between .330-.800, fluctuating between the values and mostly between .500-.800
02 Sensor between .300-.855, mostly between .400 and .800

The platinum-palladium coating in the converter won’t do its job until it’s heated up. You can’t test it cold and get anything meaningful. As a matter of fact, many new cars put the converters right up against the exhaust manifold, so that they get hotter faster, for emissions reduction.


Clearly, you are getting P0420, because the downstream sensor’s signal is pretty much matching the upstream sensor’s signal

I believe the criteria is this . . . if the downstream sensor’s signal fluctuates at 80% the rate of the upstream sensor, the code will be generated

Aged, lazy and skewed sensors and exhaust leaks do have an effect . . . keep that in mind

The downstream sensor’s signal should be very flat and steady. You don’t want massive fluctuation

The upstream sensor’s signal should be fluctuating from below 200mv to above 800mv, the average being about 450mv. That said, it should be fluctuating pretty quickly

Have you had overheating, excessive oil consumption, massive and persistent misfires . . . ?

Exactly which oxygen sensor did you change . . . upstream or downstream?

What brand sensor(s) did you install . . . ?

How many miles on this car . . . ?

Thanks for the info and clarification. Here are the answers to your questions:

Never had any overheating, change oil regularly and no misfire. The only thing I noticed was one of the spark plugs was fouled a little more than it should be. I changed plugs and wires a few days ago. Added about 250 miles since then.

which oxygen sensor did you change: Upstream

What brand sensor(s) did you install: Bosch

How many miles on this car: ~210.000


thanks for the information

Personally, I would have installed a Denso sensor, since the original sensor was quite possibly/probably made my them

At 210k miles it is quite possible that the cat is no longer capable of doing its job properly

By the way . . . you did not answer my question about oil consumption

I asked if you had excessive oil consumption, not how often you change it

Do you periodically check the oil level?

If you don’t check, and just do oil changes at regular intervals, how do you know you don’t have excessive oil consumption?

I suppose you could look at the quantity of oil drained, and say it looks about right . . .

How about this scenario . . . if you don’t check the oil level yourself, and bring the car to a shop for an oil change, then you will quite possibly not be told that the amount of oil in the crankcase was very low. Because the guy doing the oil change is probably not doing a mental calculation, to check that the amount draining out looks correct.

Excessive oil consumption shortens the life of a cat

But even if your cat is a goner, you got 210K out of it, and that’s respectable

FWIW . . . a 5 gas tailpipe test on a dyno would tell you a lot about the cat’s condition

I must have missed the oil thing. Yes, I religiously check the oil level, especially because of the car’s age. 90% of the car’s mileage is highway driven, at least the last 120K since I bought the car used. I change the oil every 3.5-4K miles, adding a bottle of Lucas oil with each change. I used to have a lot of “ticking” when starting the car in the cold. Since adding the addictive it is completely gone.

The last time I changed the oil I mistakenly used synthetic oil. Not sure what this will do in the future and whether or not I need to continue using it from now on. The car is driven 130 miles 7 days a week.

Forgot to mention.I purchased a couple of bottles of Mr. Gasket Cataclean. I figure it can’t hurt to try it in case the converter is just gummed up.

Thanks for your information.

Do you have any sense if the rear O2 sensor is original?
210K miles is a lot for an O2 sensor.

I would imagine it is the original. I bought the car with a 100K miles on it and can’t imagine the previous owner having changed it. I believe I know what you are thinking. Perhaps changing that sensor as well to take them out of the equation? Maybe my project for an upcoming weekend.


You did no harm using synthetic oil. No need to stick with synthetic from now on. You can go back to conventional and not lose any sleep over it

Anyways, I think you’re wasting your money with the cataclean . . . snake oil. If I was you, I’d return it and get your money back. And if you lost the receipt . . . or actually intend to use the stuff . . . I hope it wasn’t expensive

You’re getting good guidance here from db, however I’d like to add that with 210,000 miles on it you should prepare to change the cat converter. Try the downstream sensor, by all means, but that is perfectly acceptable performance from the converter. Don’t be surprised or disappointed if you end up changing the converter.

Converters work by making a chemical change to the passing NOx molecules that come in contact with the converter’s catalyst by stripping the oxygen atoms from the nitrogen. Over time, the catalysts becomes contaminated with the undesired components of the exhaust, like carbon, and the catalyst can no longer intimately come in contact with the passing NOx molecules to cause the change. It’s normal. How early it happens is determined by a number of variables, such as how clean the engine burns, the balance between the size of the converter and the engine size, driving habits, maintenance, driving environment, and a bunch of others. City engines in cold climates and engines that only go to the corner store are the ones most susceptible to the process, but all engines produce some level of undesirables, and most cats will go out eventually.

Thanks all for the advice. I guess I will change the other sensor and if that does not work the converter. That brings me to another question. Any special recommendation as to what brand to use? I will probably try to do it myself. I love to crawl under cars and beat up my knuckles while at the same time thinking of what would happen if the jack stands give out.


“Any special recommendation as to what brand to use?”

That depends . . .

What are your goals?

Sell the car now or soon, with no check engine light on?

Get the car through an emissions inspection now?

Keep driving the car for several years?

The answers you give will influence what kind of cat you should get

This is a commuter car, driven daily at least 130 highway miles. I plan on keeping it as long as I can. I do not have a Dollar value in mind as far as setting a limit on repairs. Everything is in decent shape and the engine runs strong with the 220K miles on it. It’s only a 2.3L Acura CL with a Honda engine, but I expect many more miles from it. I previously owned a Honda and put 455K miles on it. It was totaled in an accident.

I don’t think I have problems passing emission testing. I went about two months ago and passed with no problems (*). I reset the check engine light 500 miles ago, same time I changed plugs, wires, and all fluids/

I looked at some converters, Magnaflow, Bosle and some others in the $160-220 range and would feel comfortable paying that much for it. I don’t mind doing the work myself, I have the tools and getting a little dirty is not a problem with me. As a matter of fact I enjoy working on cars, it seems each time I do it I learn something new.

IMHO the important thing is to make sure that whatever you buy is a direct-fit, OEM-replacement converter. I recommend against a universal-fit converter, which usually requires welding anyway. Some chain muffler places (and many DIYs) jury-rig generic parts, and IMHO that’s a formula for premature failure.

There’s a lot of debate on the longevity of various brands, but the biggest premature killer of cat converters is (1) poor [generic] designs and (2) poorly operating engines. and some people believe that a good converter will survive a dirty-running engine… and it won’t.

And get all stainless steel, not aluminized steel.


I don’t have a high opinion of aftermarket bosal cats

I have a so-so opinion about magna flow cats

As mentioned, get stainless steel, although I’m not sure you have a choice. I think of late, all replacement cats are stainless steel

what state are you in . . . ?

Does your car have california emissions . . . FYI many cars not sold in california met the specs. Many states require you to install the “correct” emissions parts. That means replacing california spec emissions parts with california spec emissions parts

I’d look at your underhood emissions sticker, and check the rules for your state

Since you’re planning on driving the car for a long time, I’d plan on spending more than $220. Considerably more, actually. You want a direct fit cat, not a weld-in cat. In many states, the cat has to be approved. Savvy inspectors will look at the plate and fail you if the cat is not approved.

I may be wrong, but I believe eastern catalytic makes decent quality cats

Bottom line, you may have to do your homework and ask a lot of questions

I’d hate for you to spend money and then fail, because the cat you bought was wrong in some way

A quick fyi for the OP: The upstream O2 sensor is how the engine computer optimizes the fuel/air mixture for the engine. To assure complete combustion with an O2 sensor but no HC sensor in the exhaust stream, it has to constantly modify the amount of gas injected while trying to get the pre-cat O2 reading as close to zero as it can, but always above zero (otherwise it would be injecting too much gas). So the reading bounces back and forth corresponding to between too much gas and a too little gas. What you are seeing, that’s normal. If the cat is doing something, the post cat O2 sensor will read differently than the pre-cat sensor. If they read the same, then the computer suspects the cat is just passing the exhaust what it receives as input directly to the output and out the tailpipe, which would mean the cat is not working. Which is what I suspect in your case.