Catalytic converter vs O2 sensor

I have a 2006 Jeep Liberty with 135,000 miles. My check engine light is mostly on. Sometimes it is off. One mechanic tested the car and said the catalytic converter is bad. Another mechanic said that test can often be false positive and it might only be the O2 sensor. The difference between the two is about $1,000. Which mechanic is correct? My car is registered in a state that doesn’t do car inspections.

What is code ?


“catalytic converter is bad”

That MIGHT be correct . . . but further testing is needed

“it might only be the O2 sensor”

That might also be correct . . . again, further testing needed

I am guessing that if you check your owners manual on a 2006 OBD 2 vehicle, it will tell you that you should be changing your oxygen sensors about now. Therefore, there is no decision. If they replace the catalytic converter, they are going to insist on replacing the oxygen sensors as well or they won’t stand behind their work.

If it were me and the oxygen sensors were due for replacement anyway, I would not spend any more time and money on diagnosis until I stopped by the auto parts store and picked up some new sensors and screwed them in. If that turns out not to be the whole problem, you have lost nothing but a few minutes of your time.

You are lucky. My cars each have two catalysts and they are $1000 each just for the part.

If you live state that has escaped emissions testing, no further action is necessary…

Today’s smog cars have a rear oxygen sensor that measures catalytic converter efficiency…That’s all it does. So maybe your CAT has become inefficient or maybe the sensor generating the signal has become faulty. Or maybe a little of both…

You could spend $150 to replace the rear sensor and see if that resolves the CEL issue. If not, you can contemplate replacing the converter…Or you can put a piece of black tape over the annoying CEL and drive on…


Can you post us a link to an OBD2 car’s owner’s manual, which clearly states that the sensors should be replaced at set intervals/time frames?

Oxygen sensors are so much better than they were 20 or 30 years ago

But you’re right about one thing . . . nothing lasts forever

Personally, I think the sensors should first be tested, with a DMM, scanner, scope, etc.

@Caddyman my girlfriend then and wife now did your laise faire approach of “drive on”. Her check engine light was on for two years on & off typically in few increments. It was tied back to her o2 sensor.

So finally at 149k miles the car started to stall when going from highway speeds and onto exit ramps slowing down. She happened to break down near a Honda dealer so drove in. They called us stating we have great news!! You have $1500 in repairs for a damaged catalytic convertor and two o2 sensors. The good part apparently Honda had some sort of emmisions warranty till 150,000 miles due to TSB and all was covered. They also change her wire and plugs (she never did that) free of charge and few other items.

We walked away fat, dumb and happy paying $50 for a non covered exhaust piece they needed to complete repairs.

That all being said we address these things now dodging a bullet.

That’s a good point. If you regularly drive around with your engine light on you’ll never know if another problem crops up that may need attention soon.

There’s broken and there’s fixed. If the engine light is on, something needs to be fixed.

Fat and stupid is no way to go thru life. Sez dean wormer.

My theory is always relace the O2 sensors before the cat. The sensors are frequently the problem and cost a lot less than a new cat. Many people get burned by replacing the cat first, only to find it wasn’t the problem.

I am guessing that if you check your owners manual on a 2006 OBD 2 vehicle, it will tell you that you should be changing your oxygen sensors about now.

REALLY?? What manufacturer has a time or mileage interval for O2 sensors? I’ve NEVER EVER seen or heard of it. And I’ve yet to replace a faulty O2 sensor…even on vehicles with well over 400k miles on them.

To the original question: The only way the ECU knows a cat is bad by the readings it get from the O2 sensors. A bad O2 sensor will indicate that the catalytic converter is bad. If the code indicates a bad cat…then further tests need to be run to determine if it’s a bad O2 sensor or a bad cat. Good mechanics know how to do this.

I assume next argument will concern using oem cat or aftermarket cat and/or new oxy sensor.


My memory’s a little foggy, but here goes . . .

I remember, back in the day, some Benz models actually had an oxygen sensor light which would come on at preset intervals.

That said, those models were pre-OBD2 cars. Some of them were even pre-OBD1

Here’s the funny part . . .

On the pre-OBD1 (pre-1988) cars, after you replaced the sensor, you were supposed to remove the cluster, so that you could REMOVE the bulb

On the pre-OBD2 (1988-1995) cars, after you replaced the sensor, you were supposed to reset the oxygen sensor “relay.” However, the relay could only be reset so many times. After that, you had to also replace the relay.

I’m not entirely sure, but I heard that some other manufacturers also took this approach

I did have a problem with an O2 sensor a few years ago. But it was caused by the mechanic who replaced my exhaust. He damaged it accidentally.

I get the car back…and on the way home my engine light goes on. When I get home I looked up the code and it indicated I might have a bad catalytic converter. Knowing that bad O2 sensor can cause this…I looked up in my shop manual how to test it. From what I remember it wasn’t that difficult. Had to pull the wire off the O2 sensor and jumper it to ground (or something like that). I took the truck back the following day…and he said he might have damage it…so he replaced it for free. Sure enough…when he showed me the old one…it looked like it had been hit by a hammer.

I don’t have access to a Jeep Owners (service schedule) manual. I have the manuals for all four of my BMWs, which are all OBD2 system from 1997 and 2004. The manuals for the '97s say to replace all four sensors every 90k miles, and the manuals for the '04s say to do it at (I think) every 110k miles.

Bosch is the OEM oxygen sensor manufacturer for many domestic and import car brands, and they recommend oxygen sensor replacement for preventive maintenance at the following intervals:

Type of Car Mileage Replacement Interval Recommended
Unheated oxygen sensors on 1976 to early 1990s vehicles Every 30,000 - 50,000 miles
Heated (1st generation) oxygen sensors on mid-1980s to mid-1990s vehicles Every 60,000 miles
Heated (2nd generation) oxygen sensors on mid-1990s and newer vehicles Every 100,000 miles

I have been known to run them much longer, and asking Bosch how often to replace your oxygen sensors is like asking an insurance salesman how much insurance you need, but if you are chasing a problem, this is a good place to start.

In reality, if they replace the cat, they will likely replace the O2 sensors as well because they are right there. They don’t want to see you back so they’ll replace all the parts that could cause the code to be thrown. You don’t want them doing that.

First check for exhaust leaks - that’s a common cause for an O2 sensor related codes.
If that’s not it, follow the other guys’ advice and replace the appropriate O2 sensor (likely the post CAT sensor) first and see if that fixes the problem.
If it doesn’t, move on to the cat.

My money is on the O2 sensor or exhaust leak.


Can you please post a picture of your BMW owner’s manual page which tells you to replace the oxygen sensors at set intervals?

I have looked on this official website, and the owner’s manual I looked at (2004 530i) did not mention anything about replacing the sensors at a set interval

It would be interesting for me . . . and others . . . to see this in writing, from a car manufacturer, in recent years, that is.

Bosch is the OEM oxygen sensor manufacturer for many domestic and import car brands, and they recommend oxygen sensor replacement for preventive maintenance at the following intervals:

Of course they do…Bosch is trying to sell O2 sensors. That’s totally different then the car manufacturer recommending the replacement of O2 sensors.

I’ll bet that companies that make steering-wheels recommend a change interval also.

I have had bad O2 sensors. The heater failed.

I have only the 2004 manuals handy, and it turns out that I misrecalled the mileage, BMW recommends that oxygen sensors be replaced at 120k miles on the '04s.