Replace catalytic converter or oxygen sensor? Not sure what to do.
 

Hello,

I don't know much about cars, but I hope this is the right place to ask a question about a problem I'm having. I did search and found something similar, but not exactly the same as what I need.

Today, my check engine light came on. (The car is a 2000 Toyota Corolla.) After bringing it to my mechanic, I was told that it was the code for the catalytic converter. He said that it could possibly be the oxygen sensor, so since he would replace both anyway, I should try replacing the sensor first, and see if that alone would clear the engine light. After asking him a few questions, he mentioned that for now my only problem would be that I wouldn't pass a vehicle emissions test. Funny thing is, I actually just took my car for the emissions test two days ago and it passed. Does that mean that it is not the catalytic converter, or is it possible that it was ok then and suddenly two days later, it needs to be replaced? I figured if it weren't working or would be on it's way towards needing to be replaced, I probably would not have passed emissions, is that right?

I then brought the car to a second mechanic. He said that the first guy doesn't know what he's talking about, because if it were the sensor the code would have said so. He said he wouldn't replace the sensor at all, but just replace the converter. However, like I said, I did pass the emissions test two days ago.

They each wanted to charge between $260 to 3 or 4 hundred dollars for the converter. However, I went to auto zone and found a universal one for my car for $80. The guy there did a diagnostic reading too and it said:

P0420 - Catalyst system efficiency below threshold - bank 1. Probable cause:

1. Air leak in exhaust before rear h02s (heated oxygen sensor) bank 1
2. AF sensor error
3. Fuel system fault
4. Faulty catalytic converter

Some of the customers there gave other suggestions of what the problem could be such as needing to seal the gas tank better or something like that? I am not completely sure.

Either way, I am not really sure what to do. I don't want to buy parts and pay for labor so that the mechanics can keep trying things to decide what it is that is making the engine light go on, and they each keep giving conflicting opinions. I am going to go to a third mechanic in the morning, and maybe the Toyota dealer, but does anyone have any advice as to what it could be? Does the fact that I just passed emissions two days ago make any difference?

Thanks in advance!

I've lived with my P0420 code for several years now. You just turn it off when the light comes on. Doesn't affect mileage. I'm pretty sure if you clear the code right before emissions test you'll even pass it. Basically it's just the redundant 02 sensor saying it's not reading exactly as it should, but it COULD also be the cat/front sensor. I say find a way to turn it off (unplug the battery if you don't have a code scanner/scangauge) and see how it affects your mileage. If there is no real change (I know winter is not nearly as consistent for me) then it's nothing to worry about, really.

PS: All this info is based on my own experiences with subaru, but an engine is an engine.

How many miles are on the car?

Do you have an exhaust leak?

I am not a mechanic just a tinker.

That is the problem OBII codes. They don't always tell you what to replace. But what the system "thinks" the problem is. If a sensor is reading incorrectly it may indicate a different problem an O2 sensor indicating a cat problem. This is were a good & trusted mechanic comes in. He must make decisions, do I replace the part or or spend the time trouble shooting, which is more cost effective.

The advice he gave is quite reasonable in my opinion, based on just doing a code check. You didn't state how many mile are on the car if your car is routinely maintained. My self I usually just reset the code and see what happens, the proceed.

There are about 111,000 miles on the car. I don't think there is any sort of leak, but I will make sure to double check that today. As far as being maintained, I have the oil changed and fluids checked and things like that, but that's mostly it. I bought the car used about a year ago. I do have to add oil myself very frequently, about every 2 to 3 weeks. It burns oil like crazy. I found this out about a month or two after I got the car, when I started to hear knocking coming from the engine, and realized that there was not a drop of oil left inside. I looked it up online and it seems that many people with older corollas have the same issue.

So, you think it's ok if I just ask the mechanic to shut the light off and ignore it? Does the light keep coming back on?

The oil burning is the culprit. Oil is being burned, and it is going out the exhaust, and coating the catalyst, and the oxygen sensor. Converters do not ususally FAIL for no reason. Oil, coolant, or excess or lean fuel conditions send contaminants into the system.

This is probably not going to get better. You can replace them both, but the burning oil is going to continue to contaminate the converter.

Possible short term is to use a spacer that pulls the oxygen sensor back a bit out of the direct exhaust flow, to minimize the oil coating. The converter is going to continue to fail. Best case is that the smokey exhaust has coated the honeycomb inside, and the exhaust merely flows through, unobstructed. It is possible it could start to build up on the honey comb material and impede exhaust flow, making the engine work harder to get the exhaust out.

You can consider fixing the problem (another engine, or engine rebuild) or cut out or gut the catalytic converter (it is not doing it's job due to contamination). Gut or cut out is technically against the rules, but if it gets clogged you are going to lose efficiency and MPG. Unless you are willing to fix the basic engine problem, eliminating the converter makes some sense as it allows you to be as efficient as possible, consider the burning oil situation.

Unless you have the ability to get into the computer code and shut off the oxygen sensor ( I presume this is the 2nd sensor that tattles to the computer how well the catalytic converter is doing to clean the air ) the code will keep coming back.

The upstream 02 sensor is one of several sensors that controls the air/fuel mixture and has nothing to do with the PO420. The downstream 02 sensor takes a reading of the pollutants post catalytic converter telling the ECU the efficiency of the converter. If it's throwing this code usually it's a bad converter. Was the check engine light on when you had the emissions test? Probably not unless they just do a tailpipe test and it passed by the skin of it's teeth. If the emissions testing equipment is hooked into the OBD II connector and the CEL was on the ECU would have sent a message to the testing equipment telling it there was a problem and the car would have failed emissions testing. If the light just came on after the emissions test then the converter was probably removing enough of the pollutants at the time to barely meet the standard, but the converter efficiency dropped shortly afterward turning on the CEL. Unhooking the battery for a few seconds will turn the light off, but if it's a problem with the system it will probably be back on within 50-150 miles and if they hook up to the OBD II port when doing the emissions test and the battery has just been disconnected the ECU will also tell the testing equipment that all the systems aren't ready for emissions testing which usually takes 50+ miles for everything to recalibrate. I agree that the oil burning could likely be what is causing the problem just as explained in one of the previous posts. Also on OBD II systems if the converter is working correctly for a certain number of starts while the CEL is on the light will go out by itself without having to disconnect the battery or erase the ECU's memory with a code reader. This is not only true with this code, but any code.

Actually, if you're burning that much oil there is a good chance the Cat actually is damaged, whether that matters is up to you, if it passes it passes. My code is indicating a fault, but it is actually the sensor being a baby.

If you turn it off, it will likely come back on if something is actually wrong. Cat's don't fail unless something else is going wrong as well.

The inefficiency of the catalytic converter won't affect your fuel mileage unless it's stopped up. Back in the 70's & 80's when it was still legal to remove them from your own cars I removed them from nearly every car I owned and burned leaded gas. Some people used to say removing the converter would help fuel mileage, but I never saw an increase in mileage it was just that leaded fuel was less expensive than unleaded. This was back in the days when $.60 a gallon was highway robbery. Probably 75+% of the members on this forum don't even remember those days, if they were even around they were still sh1tting their pants.

Thanks all for the replies and helpful information.

So it seems then that it could be that a year of oil build-up could have damaged the converter. I am not going to replace the engine as it is not worth putting that money into my car, so if I decide to replace the converter or sensor, I guess this could just happen again. Is it possible, then, to bring the car in for some sort of engine or exhaust cleaning every so often so that the oil doesn't continue to build up?

As I see it now, my options are to either just ignore everything, replace the sensor, replace the converter, or replace both. If I were to ignore the problem, and the converter became clogged, what happens besides lower gas mileage? It is illegal, then, for my mechanic to just remove the converter, is that right?

I would rather not ignore the problem though. Would you guys suggest replacing the converter first, see if the light goes away, and if not, replacing the sensor as well? Or vice versa, replacing the sensor first? Ford Man, you mentioned the light going out if the converter is working properly, after a certain amount of starts. Does that mean if the parts were replaced, the light wouldn't shut off right away, even if that part was the cause?

The check engine light was not on during the emissions test. It came on two days later.

Sorry for all of the questions, I only learn about the parts of cars and its system, it seems, as the problems related to them arise.

If you replace the parts with out finding the cause of the oil usage, your are domed to repeat the process. The question being how long it will last before the problem returns. Are you require to do an annual emissions inspection? Are you able to do any of the work yourself?

The light is on because the sensor behind the converter is not seeing enough change in the "cleanliness" of the exhaust. Basically, the sensor behind the converter is a tattle-tale. The computer sees the difference between the front sensor reading and the back sensor reading, and if it doesn't see a change, it throws the code saying the converter is bad.

I wouldn't replace the rear oxygen sensor. It sounds as if it is doing its job and reporting. There are DIY and HOW TO articles on how to clean an oxygen sensor, though I have never done so.

Options:

• replace the converter, though I expect the converter will fail again, dependant on oil burning. The 80.00 universal converter doesn't sound too bad, but you might consider setting things up so you can do it yourself down the road, when the replacement converter dies. How often it will have to be replaced at 80.00 per could affect decision.
• leave the converter, and watch performance and MPG to see if the converter is becoming clogged.
• gut or remove the converter, not totally legal, but you would ensure it is not becoming a restriction and causing further problems or hurting efficiency

Here is what I would do:
Buy the replacement converter, and have a muffler shop weld 2 flanges onto the end of the converter. Have them also weld 2 flanges into the exhaust. Have them also weld up a straight pipe with the 2 flanges. I would keep the converter for next year when you have to have the test, and bolt it in place of the straight pipe that you run daily.

While I acknowledge that a converter does the job of cleaning the exhaust for better air, I also take into account that the car is already burning oil. The converter is not going to fix that. For me, my comfort would come in knowing I am trying to be as efficient as possible, and while the car is putting out emissions, having a converter on there that is contaminated and not able to do it's job, is not really a benefit to the equation, and more of a hindrence, as it impedes exhaust flow, especially as it get more and more clogged with oil residue. I'd buy one more converter, run a straight pipe, and put the converter on before the test so it passes.

A muffler shop CAN NOT remove the converter. Removing a catalytic converter is a federal offense and punishable by up to a $25,000. fine and up to 5 years in prison. This isn't like it was back in the 70's and 80's, even then it was illegal for a shop or garage to remove them, but you could remove them yourself from your own car. If the converter gets completely stopped up the car will have very little power. I still have my doubts that the 02 sensor is playing any part in this, but you can go on ebay and buy universal 02 sensors for replacements. You have to cut the plug off your old sensor and splice it onto the new sensor, but the last time I looked they were only about $20. instead of $75+ like a new one with the plug installed. I used some universal sensors several years ago and they functioned fine. Just be sure before you order one to check to see how many wires your sensor has, some have 1 wire, some 3, some 4, and some 5. Most cars have a 4 wire sensor which is one ground, one signal to the ECU and two heater wires. Just match the colors except the two heater wires will be the same color and have to be configured the same way they were on the old sensor when installing the plug. Even if the problem is corrected and you don't reset the computer by removing the battery cable or with a code reader it will take a while for the light to go out on it's own. Oil will ruin the catalyst in the conveter, but there would have to be lots of oil getting into the exhaust to be the full reason for the failure. If you decide to replace the converter, shop around online and on ebay and you still may find a better price than $80. There's equipment for testing the catalytic converter, but I don't know who would have it or how expensive the testing is. Since you just got the car inspected you will be OK until the inspection comes around next year, but at that point it's not going to pass with the CEL on. I just remembered a friend of mine that's been a mechanic for several years told me you could clean an 02 sensor by soaking it in Sea Foam overnight. I've never tried it so I don't know whether it works or not, but even if it didn't it won't hurt the 02 sensor. To remove the 02 sensor you will either need an 02 socket or if the plug will fit through the box end of a 22 MM wrench just run the plug and wires though the wrench where you can get onto the 02 sensor with the box end. Depending on how much working room you have you may need a stubby 22 MM wrench. Also be sure to put anti seize on the threads of the sensor when you put it back in to keep the heat from seizing it where you can't ever remove it again.

Yes I am, and I am hesitant to do anything more than very beginner by myself.

Maybe I will do this for a bit first, and if I notice a drop in performance, will replace the converter. I'm also going to check the oil even more often, so that it doesn't run low.

As far as pickup and mileage? It would be noticeable, right?

But using a code reader, could the mechanic tell right away if he replaces the part if it was the part causing trouble? Or would I have to drive around a while first?

Thanks again for all of the replies and help.

If I were short on cash ( who isn't ) and not too good with a wrench, I would also take the wait and see approach while monitoring the mileage and performance. That way the closer you get to next years inspection before you replacing the parts the more likely you will pass your next inspection.


Sometimes a code will reappear after several days. I had a car that made about 7 trips to two different shops before it was fixed. It wasn't the same problem you are having though.

Good Luck and hope to see you back here in other posts.

Yes


Even if the light is turned off by a code reader the ECU has to relearn all the engine perimeters and at that point it will turn on the check engine light if there's still a fault. This process will usually take anywhere for 50-150 miles of driving. I know this isn't what you wanted to hear, but better to be informed than left guessing. The only sure fire way to find out if the catalytic converter is the problem is either throw parts at the problem (gets very expensive) or find someone that has the testing equipment for the catalytic converter.

Check engine codes do not tell you which part is bad, they tell you what's being affected. Just for the future reference.

This may be true in some cases, but PO420 usually does indicate a bad converter. I think the the definition for the code reads something to the effect of "insufficient catalyst efficiency" meaning the 02 sensor detects a problem with the polution in the exhaust gasses. The codes are there to steer you in the right direction of things to look into. Yes, it's possible to have certain codes that will give you several things to check that may not be the fault, but could contribute to the problem. Several years ago I took a car in for emmisions testing (tail pipe sniffer test) and the readings maxed out the emmisions testing equipment (pre OBD II) and the problem was a bad vacuum line.

I did some work on my girlfriends car a few months back, she also has a Toyota Corrola with the same P0420 check engine light code.
First thing I did was checked the gas cap, the seal on the gas cap was bad, 2nd thing I did was had her only run non ethanol gasoline.
3 months or about 4,000 miles later and her check engine light has not come back on, her car is still burning oil but it seems to be burning it and not damaging anything, so that would be my advice, use better gasoline and install a new gas cap as they have a check valve in them that can wear out.

Some of us are not lucky enough to have a choice between straight gas or ethanol laced gas.

hey carwhat just my two cents but fordman is giving you the best direction on fixing your problem you can also ohm test an o2 sensor but if its oil coated you need to pull it out and clean it or replace it you will need probable need a special wrench to remove it you can borrow from autozone and i believe advance auto dont through parts at it find the problem first (usually)