Oxygen sensors


When the check engine light came on a few months ago on my 1997 corolla, I had the catalytic converter replaced. I was warned that the oxygen sensors would be next. Sure enough, last month the CEL lit up again. The front sensor was replaced last week. I didn’t even drive 20 miles before the light came on again. It’s still cheaper than a new car, but will replacing the rear sensor be the end of this annoyance, or do I just ignore the light?


How many miles on the odometer? The O2 sensors tell your engine’s computer whether it’s burning fuel efficiently, sensing oxygen in the exhaust indicates an incomplete burn. When they “go out” they’re telling the computer erroneous info. and your car runs inefficiently. Your owner’s manual probably has a schedule that includes when to change them. Are you past that time? In these days of increased gas prices, efficiency should be a top concern. Yes they’re expensive, but that cost will be returned at the gas pump.
As your car ages, other things will begin to breakdown. If your car is still in good condition, pay for the repairs and keep on truckin’. When the repairs become more than the car is worth, a new (newer?) car is in order.(IMHO)


If you are concerned about pouring a lot of money into a car that will fail soon or will be a money pit talk with your mechanic and evaluate the car like you were thinking of buying it used. I hope that you have a trusted mechanic.

The front oxygen sensor monitors the burning of the fuel and allows adjustment of the fuel-air mixtire. The rear oxygen sensor monitors the catcon efficiency. I can’t see your car, but my SOTP advice is to put in the rear oxygen sensor and drive on, IF that is the real problem. It should not be that expensive. Oxygen sensors are usually not on a replacement schedule, but are replaced when they trigger a trouble code after they get “lazy”

I am a little troubled that the CEL lit up only 20 miles after replacement of the front oxygen sensor. It indicates the possibility that the codes were purged at the time of replacement and the same code(s) came up. Do you have the trouble codes from when the front oxygen sensor was replaced? How about for the current problem?


What caused the catalytic converter and oxygen sensors to go bad? Do you have any idea? Whatever it was, will cause the replacements to fail, sooner or later.

Check upstream of the catalytic converter. The engine has everything to do with the health of the catalytic converter and oxygen sensors. If the engine isn’t operating at optimum efficiency, it will “poison” the new catalytic converter and oxygen sensors.

The repair manual covers how to check, and maintain, the health of the engine. Save those part numbers for the catalytic converter and oxygen sensors for when it comes time to order them, again.


I’ll second Hellokit. Failures have a root cause and JMHO here, but a converter and even the O2 sensors should last the life of the vehicle.
The number of O2 and converter failures posted on this board sometimes amazes me.


It’s got a bit more than 117K miles on it. It’s been relatively trouble-free until the last few months, so I count myself lucky. I’m hoping to keep driving it, even though it is without name or gender, but previous experience has shown me that one fix often brings on the next problem. I


The upstream O2 sensor caused the cat failure. The downstream sensor is a self test of the emissions system. Bosch recommends change at 100k. O2 sensors are consumables, not life time. I recently replaced the Bank 2 Upper O2 sensor on an 05 Duratec at 25k miles because the voltage wasn’t where it should be. Cost of the sensor was $42 delivered. Took 45 minutes to install. Cheaper than a cat by far and will improve mpg. I will replace downstream sensors only on failure.


little tip,watch the fuel trim values(means alot)

good luck


Explain how the upstream O2 sensor caused the cat failure.
Changing parts when they test faulty is not the same thing as changing a part based on only a symptom, or an arbitrary mileage figure. Parts which suffer from mechanical wear are NOT in the same boat as sensors and other electronic parts.


Welcome to Toyota World!!! At that mileage, I’d have changed the O2 sensors twice. Have you also changed the timing belt, or did Toyota finally get some sense and go back to timing chains? Seriously, the O2 sensors are consumables as are catalytic converters. In plain talk, they wear out and need to be changed periodically. Some parts are expensive, some are not. I changed the O2 sensors in my Jeep Cherokee last year to the tune of $430.00 for all FOUR!!! I’m laughing all the way to the bank as gas prices soar! My inline six gets 23 mpg CITY!! Keep up the periodic maintenance and your car should perform nearly as new. Yes it’s costly at times, but you get paid back by driving an efficient car, not having loan payments and not paying for full coverage on a car that’s no longer in the NADA Blue Book.

BTW My Cherokee is known as "Sabrina".  My dad told me long ago, give your car a name and you'll soon treat it as a person instead of an object.   People, you take care of, objects just get used.


I have to respectfully disagree with the suggestion that 02 sensors are consumeables and should be replaced on a regular basis along with disagreeing that an upstream O2 will kill the cat.

SAAB and VW started using Lambda controlled FI systems back in 1977 and at the time SAAB stated that the O2 sensor should be replaced every 15k miles as part of the recommended maintenance. Since the sensor was 400 bucks at the time (figure that in 2008 dollars!) the car owners were a bit irritated to put it mildly. SAAB then said change them every 30k miles and this did not go over well either.
Eventually SAAB stated to test them at 60k miles and replace them only if necessary and it was seldom necessary unless there was a problem that contributed to killing the O2.

I’ll say this. In regards to both mine and my family’s cars I’ve never, ever had to buy, replace, or even need an O2 sensor (or converter) on any of our cars except in one case. In that one case I was doing some maintenance work underneath and a wrench slipped. It banged into the O2 ceramic and cracked it.
It was still operative and left alone until the next visit to the Pull A Part yard where another O2 only costs me 5 bucks.
As far as O2 failures, never a one even in mileages ranging up to 300k or even 400k miles plus.

Besides, an O2 is not guesswork; they’re easily checked with a digital VOM.


Sorry, I don’t know what a fuel trim value is. Would you please explain?


The upstream sensors signal by voltage the amount of oxygen in the exhaust stream. The PCM then commands the amount of fuel injected. If the sensor is deteriorating due to age or contaminants, excess fuel is injected. The cat overheats due to the excess fuel and eventually fails. O2 sensors cycle rapidly rich/lean. The average voltage should be 0.45V. I read voltage and LTFT from Car Chip. That’s how I know. The other very good method is dynamic tail pipe testing for O2, HC, and CO. I use those tests also. Your analogy mechanical and electronic parts is absurd.


I have to respectfully disagree with the suggestion that 02 sensors are consumeables and should be replaced on a regular basis along with disagreeing that an upstream O2 will kill the cat.

I agree completely.

This “wisdom” only applies to OBD I vehicles, and isn’t precisely true for them either. An oxygen sensor can get far enough out of range to affect fuel economy and driveability without tripping the CEL. But that’s not exactly the same as changing them on a routine basis when there are no symptoms present. An oxygen sensor far enough out of range for you to notice the effects will almost always trip the CEL in an OBD II vehicle like the OP’s.

I’ve also never had to replace an oxygen sensor or cat on any of my vehicles, many of which I drove in excess of 150K miles.

Besides, I thought curiosity killed the cat…


Seriously, the O2 sensors are consumables as are catalytic converters. In plain talk, they wear out and need to be changed periodically

I guess they do wear out…but I’ve NEVER had to replace either on any vehicle my wife or I owned. We averaged well over 250 miles before we traded/sold them.


After 20 years plus O2 sensors and PCM’s, amazing that so much ignorance of a Bosch invented system exists.
Here’s a primer link:


I don’t think of O2 sensors as consumables either. I can see how Bosch would, they sell them. But I have had them go bad also. Not so often in the “old” days but the newer sensors are heated and the heater circuits tend to fail. Our Honda has eaten three of them, 140k miles, at Honda’s expense though. My Nissan truck has eaten one so far, 150k miles. None for the Saturn yet, 164k miles. The only non heated one I’ve had go bad was in the 86 Toyota, but it lasted about 250k miles, replacement was cheap and that was recovered in fuel savings.

For the OP, is someone checking the codes before doing this work? Do they show you the readout? There are a lot of things that can cause a CEL besides catalytic converters and O2 sensors. A loose gas cap or overfilling at the pump can lead to this light, as can a bad spark plug or any number of other things. A good diagnostic is in order here.

Most auto-parts stores will read the codes for free and you can see the code for yourself. They provide an educated guess as to the culprit part, but post the code here for additional information. It maybe necessary to go to a specialist and have a full diagnostic done for a difficult problem, but that will be money well spent. Guessing is a LOT more expensive.


Congrats on your 20 years. I started messing with them in 1977 when the Bosch O2s first appeared on SAAB and VW. Does this mean I’m 11 years up on you and still ignorant or what?

Read my posts again. I ssid nothing about blindly ignoring an O2 and/or failing to replace it if necessary.
I simply said they’re not as failure prone as some believe, that I’m amazed at the number of O2/converter complaints on this board, and that an O2 is easily tested if one is suspected to be bad.