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Ok thank you I may or may not do that. I might just get it replaced and be done with all this hassle. It's so hard to know who to trust and who to listen to when you don't know a ton about cars. I'm not really sure how to do an emissions test by itself without doing the yearly renewal registration. And again they said if it was the O2 sensor as the problem it would have brought up a different code than P0420. Plus they mentioned in this case with the P0420 code they wouldn't know which 02 sensor to replace and I could pay even more money with them trying to guess at it. Also, do you mind answering these other questions I brought up in my last post?
"You mean ask them to disable the engine light before I go on my trip or when I actually take it in for the main repair? They mentioned they had time tomorrow morning only to try resetting check engine light but also said it would probably just come right back up (they don't have time tomorrow to fit me in to do the repair).
Is there any difference in driving with the check engine light on for this code or do I need to have it reset/cleared in order for the driving to be safe that you mentioned? Because if the check engine light being on makes no difference I might not go by their shop tomorrow morning in order for them only to "reset" the check engine light.
Also, how long do you think I have until I need to get this fixed? Is the 2000 miles until my oil change too long of a wait?"
Nine times out of ten it's the sensor behind the cat. Any difference in temp between the front of the cat (where the other O2 sensor resides) and the output of the cat will cause the second O2 sensor to throw the error code.
I've seen it before. I've known people who had their cat replaced only to keep getting the code. They replaced the rear 02 sensor and the problem went away. You can easily check if the sensor is working properly using a digital volt meter, something any competent mechanic should have and know how to use.
Why would they keep getting the code if they replaced their CAT? Because I thought from what I understood changing out the CAT changes out all the O2 sensors for new ones as well? That's what I thought I was told. Because that's why they recommended just changing the whole CAT out rather than guessing which O2 sensor it is which could be more costly. They said each O2 sensor for them to replace is $250 each so that's why they recommend changing out the whole catalytic converter because it replaces the O2 sensors along with it. How many O2 sensors is there on my car by the way that they are referring to?
Ugh this is so stressful...I don't like telling them how to do their jobs especially when I have no knowledge on cars. Don't know what to do. Again they just kept saying that if it was the O2 sensor that it would bring up a different code. They said they'll do tests and won't just completely change out the catalytic converter right away but I'm not sure. So am I right in assuming when you replace the catalytic converter it also replaces out new O2 sensors? Again I don't know cars and that's what she and someone else seemed to imply?
In all fairness to this local family mechanic, there have been repairs that I've been told that needed to be done by other mechanics that this local family mechanic business said didn't need to be done. So they aren't always just trying to take my money it seems.
Last edited by jagsfan05; 12-20-2018 at 05:02 PM..
I think you may have to check your car by ANOTHER car shop or two. Just to get a second opinion. Your mechanic seem to not know much about the O2 sensors and catalytic convertors.
He is just trying to get the money from you and do the most expensive option.
Catalytic converters are mainly designed to reduce immediate, local air pollution - dirty air where you're driving, and they're very effective. But you need to remember that they reduce emissions rather than eliminate them completely.
Back to OP question - they are needed and helpful to reduce emissions, but they are not a crucial part of a car, and you can drive even if it's not working. However, I noticed that you live in California, so you need to obey local laws. Your car might or might not pass the next emission inspection - you need to check on that as advised in the above posts.
I would check the emission first and according to the results replace the sensor (starting with the sensor behind the cat) OR the converter.
In that order.
I think you may have to check your car by ANOTHER car shop or two. Just to get a second opinion. Your mechanic seem to not know much about the O2 sensors and catalytic convertors.
He is just trying to get the money from you and do the most expensive option.
Almost always this is a loss of efficiency from the cat, not a 02 sensor. The 02 sensor is measuring the delta between the input and the output of the cat. Doubt the tech is going to send his kid to Harvard on the $150 in labor to weld up a new cat.
Good Lord.. There's a mixture of right, half truth and flat out wrong info here.
So.. Let's see if I can nail this fully accurately. P0420 is a Cat below efficiency error. There is no turning the light off. You can reset the code, it will come back.
If your cat were clogged, you would notice performance issues. They might be minor, but they'll continue until basically your exhaust is plugged, and you will go about 40mph top speed. Think of running as fast as you can with a sock stuffed in your mouth. That's basically what happens to your car with a clogged exhaust. You can still run, just not very well and certainly not at your peak. I guess, more accurate would be with a sock up your butt, however, while funny, you don't rely on your exhaust all the time like your car does.
There are other things that can happen, so, that's not a blanket statement.. The catalyst can become unmounted.. At which time you'll hear a horrible rattle at idle that goes away under throttle. This usually slowly gets better as the catalyst breaks up and gets ejected out the muffler. Not only all the above.. But, let's say there's a hole in your exhaust before the converter or before the downstream O2.. Boom, P0420
But.. End of the day.. You're likely safe to drive with this code on, if you're not noticing a performance issue. The problem, IMO with doing that is.. Now you're used to the light being on.. What happens when it throws ANOTHER code on top of that? Doesn't turn the light on brighter. So, if you can and wind up doing this.. You have to get the codes read every so often to see if there's something new. I don't recommend riding with the light on for this reason.
Someone mentioned you can go aftermarket on a replacement cat.. They MAY be right. If you're in California, Maine or New York.. They are not correct. (Technically here, correct would be you can go aftermarket, but the aftermarket cat must be CARB certified) In those states, if the car came with California emissions (Which it did) then the cat has to be replaced with a CA emissions cat. These are known as CARB cats (California Air Board or something like that). Obviously, CARB cats are significantly more expensive. You didn't mention your vehicle, but.. Let's just pull a random 2010 Chevy... Say a Malibu with the 2.4L 4 cylinder. the first thing to remember.. This vehicle has 2 catalytic converters. As do most vehicles from 2000 onwards. One is mounted on the exhaust manifold normally, and is often referred to as a pre-cat(Often massively expensive because to replace you have to replace the entire exhaust manifold). The other is the standard massively expensive (usually) catalytic converter.
Rockauto has the main cat ranging in price from about $130 to $200. But, note that EVERY ONE OF THEM states "Not Legal For Sale Or Use In The State of California; Not Legal in NY or Maine For Vehicles With California Emissions". So, if you're in one of those states.. I'm finding the part, CARB certified, to be about $600.
Now.. replacing it. In general, it's a few bolts. should take an hour at most, right? Look at the exhaust system sometime. The heat, the corrosion.. Guaran-damn-teed just about that one of those bolts, at least, will snap off. Out come the cutting torches and hammers. So, you almost have to budget 2-3 hours. If you're lucky, you come in under that. So, add markup.. You're at a $1000 job no problem.
Whether this is a cat or O2 sensor problem. Being you're at 160k miles. I would lean towards the cat, but it's really about just gut feeling on my part, no science to it at all and probably not much accuracy.. jeffrow above oversimplifies things. Not wrong, just.. The O2 sensor is not measuring the delta of input and output. What's more accurate is the delta between the upstream O2 sensor and downstream O2 sensor is a determining factor in setting the P0420 code. I won't call his answer wrong, just.. A bit too overly simplistic.
A solid mechanic is going to run the car and look at the output of the upstream and downstream O2 sensors to determine if this is an instrumentation (sensor) problem, or an actual problem with the cat. They will also often measure the temps into and out of the cat. As you should pretty much always have a higher output temp than input temp. They will inspect for exhaust leaks, because THAT can cause the code to be set.
Someone mentioned using a multimeter.. No. they're going to hook the OBD2 scanner up and read the values. While you COULD do it with a meter.. You'd be crazy to do it that way when you have a OBD2 scanner available, which your mechanic had dang sure better have.
Now.. Let's tackle the question of whether you CAN run with this outside of the mechanical. If your state has emissions checks.. It's iffy. MANY (but not all) states with inspections.. If the light is on.. You fail. And don't let anyone give you the BS that you reset the light and take it in. There are monitors that tell whether the tests have been run.. If they haven't, it's an auto-fail. If they have, the light will be on, which is once again, normally an auto-fail.
Someone mentioned having an emissions test done.. There's logic to that. BUT.. If it fails, then you have X amount of time to fix it before paying for the testing again. Those tests aren't all that cheap. And, most likely, regardless of whether it passes, the only thing it's told you is whether the cat is bad. You still have to fix the problem causing the code to be set because the light being on is a fail. So.. Certainly valid, but I think there's better ways to do this.
So.. Is it safe to drive your vehicle? Short of acts of God or very, VERY rare situations.. Yes, so long as you're not seeing a performance issue.
Do you have to get this fixed? Depends. Don't know where you are. here in SC, I would not be required to get it fixed. If you're in CA, yes, and the cat (if it's the problem) will be expensive.. If you're in VA, yes, you'll have to get it fixed, but you can use aftermarket non-CARB certified parts. unless VA has changed the law recently and I don't know about it. And, being even MORE overly technical.. Here in SC, I would be required to have it fixed, because it's federal law that the car has to have a cat.. But since we have no testing, noone would ever know it wasn't working.
Is it the Cat? Possibly. At your mileage, there's a stronger probability than a vehicle at 60k, but for certain? No. It would not surprise me to be an exhaust manifold leak. Which.. Sucks because it has the pre-cat integrated to it, and that manifold is probably more expensive than the damn main cat (I didn't look at the pricing on it, so, could be wrong there)
What you should do, my opinion.. Take it to a mechanic that can tell you why the work they're saying needs to be done, needs to be done. If they say "It's got a P0420, so the cat needs to be replaced".. Run. If they say "It has a P0420, I measured the O2 sensors, they seem to be working properly both when the vehicle is hot and cold and the input temp of the cat is higher than the output." That's a pretty good diagnosis. Hopefully they've done more than that, but, they also don't want to overload you with technical details.
I'm sure i've left something out here, or something that can be clarified.
And, I just noticed your shop talking about "Guessing" which O2 sensor.. Run like hell.
Another thing I should mention.. 2 O2 sensors? True on a 4 cylinder. If you've got a V-6 or V-8.. You've likely got 4. 2 upstream, 2 downstream.
And.. 160k miles is also high time for O2 sensors to start to fail. Now.. GENERALLY that would throw a code to the O2 sensor, but.. Not always.
My end recommendation.. Go to another mechanic. A better one. Ask to see their OBD2 tool. If it's the same little piece of crap that you saw used at Autozone.. Go somewhere else. Though, honestly, some of the bi-directional tools nowadays are simply tablets. And there's at least one that is bluetooth to a phone. And I'm not talking just a code reader. Full bi-directional controls.
One other thing I'll mention here. The codes that are thrown are a starting place, not the end. You must find a mechanic that understands that. I guess you could technically say they're the end and you have to work your way back to the start, but.. Whatever. You don't read the code and replace the part it says without question. that's the point.
I think I said that but not in so many words. This is a great explanation and OP should proceed as advised here.
Start with going to another shop, OP. Your "trusted" mechanic isn't worth a dime.
Aren't there some places that will ground your car, if it fails an emissions test?
Virginia will. Fail an emissions test and you can't register your car. Convenient, since the only time you need emissions is every 2 years when your registration is due.
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This is a great explanation and OP should proceed as advised here. 
