Replace catalytic converter or oxygen sensor? Not sure what to do. - Fuel Economy, Hypermiling, EcoModding News and Forum

carwhat · 12-30-2010, 04:54 AM

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!

brucey · 12-30-2010, 05:38 AM

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.

Daox · 12-30-2010, 08:18 AM

How many miles are on the car?

Do you have an exhaust leak?

nemo · 12-30-2010, 10:54 AM

I am not a mechanic just a tinker.

Quote:

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.

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.

carwhat · 12-30-2010, 01:40 PM

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.

carwhat · 12-30-2010, 02:34 PM

Quote:

Originally Posted by brucey

I've lived with my P0420 code for several years now.

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?

Backtobasics · 12-30-2010, 02:39 PM

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.

Backtobasics · 12-30-2010, 02:41 PM

Quote:

Originally Posted by carwhat

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?

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.

Ford Man · 12-31-2010, 01:25 AM

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.

brucey · 12-31-2010, 01:40 AM

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.