Oxygen sensors or catalytic converter

Recently it seems everything has been going wrong with my car. Last week the engine light was on. I took it to the shop I’ve been using (found on CT website) and was told I need a new catalytic converter. I’ve given this shop a lot of money over the last two months and started feeling like some mechanic’s kid is going to have a great Christmas because of me. So I took it to another shop. Was told the computer is saying catalytic emissions are an issue but what I need are new oxygen sensors, because it’s very rare for catalytic converters to go bad.

Now the car is 10 years old and has 167,000 miles on it but I’ve been religious about maintenance. But I know that things go bad with age (I’m getting there myself).

So I ask the Car Talk world to weigh in.

Do I just need to oxygen sensors ($525 to replace) or a catalytic converter ($1200 to replace)?

O2 sensors should be more like $80 each. I don’t know if yours has two or four, but they are usually not too hard for you to replace them yourself. Most auto stores will loan you the tool required.
I would replace the O2 sensors first. At that mileage, they may be worn out. Even if it turns out that you need a new CC, I would want new O2 sensors in there protecting the expensive converter.

I don’t think that I would want to give any thoughts on this without actually having the codes. The engine light came on b/c the computer detected a fault or something out of spec in a system. The code(s) give you a starting point for troubleshooting the system. Someone needs to actually find out what/where the problem is rather than just replacing parts.

Look at your invoices or call the shops and find out that the computer codes were (“P” numbers like P0123).

If you can’t get them, may auto parts chain stores such as Autozone will read themn for free. Get them and post them.

If the emmission system has a fault usually the downstream o2 sensor fails and the cat converter fails eventually.

How long has the Check Engine light been on?

A look at the O2 sensor signals with an oscilloscope will give the definitive answer about what’s good or bad, but it can be hard to find a mechanic that knows how to do this AND is willing to put in the extra effort.

One of the better scan tools, which any repair shop should have, can show the voltage signals, from the oxygen sensors, that the engine computer sees. Experimental parts changing is not necessary; but, I understand that it’s fast, easy, money for a repair shop to simply replace parts. Diagnostics doesn’t pay so well, and needs more expertise.
The oxygen sensors costs roughly $100 each, one front, and one rear, equals two. See autozone.com for specifics: http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?categoryDisplayName=Routine+Maintenance&fromType=parts&fromString=search&parentId=42-0&currentPage=1&filterByKeyWord=oxygen+sensor&isSearchByPartNumber=false&navValue=14200117&categoryNValue=14299999&fromWhere=&itemId=117-0&displayName=Oxygen+Sensor&searchText=oxygen+sensor

I took it to two shops - will get codes from both places to see if they match and for further advice. Thank you.

Good advice from all respondents. You may have either two or four oxygen sensors. They generally cost $60 to $80 each for brand name sensors if you buy them on line. Replacing them is an easy do-it-yourself job if you have a set of ramps, unless your car happens to have them in a really inconvenient place.

If these are your original sensors at 160k miles, they are not worth investing the money in testing. Just put in new ones.

Yes, when sensors get old and start to respond too slowly (which you can only determine with an oscilloscope) an OBD II computer will often set a “catalyst below efficiency specification” code. Catalysts rarely fail unless you drive with a misfire and overheat them. I routinely drive my cars over a quarter-million miles (two of mine are pushing 300k right now) and I have never replaced a catalyst.

On the invoice from the first shop is code P0420. It’s a 99 toyota corolla.
In looking around online I’ve discovered that the code means Catalyst Efficiency Low. Also, looking online (at site other than CT) the majority of people are saying replace oxygen sensor’s first.

I just don’t want to replace the sensors and then have to replace the catalytic converter too.

Have a look at this and see if it gives you (or a mechanic) some ideas on how to proceed: http://www.obd-codes.com/p0420

I have a 2004 Sienna XLE and 2 weeks back had similar problem (Engine, VSC, and Trac Off lights came on and stay on). I took it to the dealer yesterday. They said engine diagnostics revealed a P0420 code and that I need to replace the catalytic converters (there are 2 of them) along with a necessary hardware. Parts and labor of about $1800. My van has 102000 miles, so it is out of warranty. I was surprised to hear from the dealer that the catalytic converters do go down in efficiency over time and they claim this is a standard problem. I am skeptical. Just wanted to hear opinion from the experts on this discussion board or other customers with similar problem. Would you recommend I rather get a second opinion? The dealer did not mention anything about oxygen sensor.

Appreciate thoughts / advice.
Thank you.