My check engine light has come on 2 or 3 times over the last year, but it usually goes away after a day or two.

I'm waiting in line to get my emissions test done this afternoon, and just as the car infront of me pulls in to take the test, I notice my check engine light suddenly came on. Unbelievable timing.

So these fools tell me I have to replace my oxygen sensor. I'm hoping the light just goes away and I can quickly get the test done before it returns.

My mileage doesn't seem to be affected, how important is it that I replace the sensor if it only seems to be faulty occasionally?

Thanks guys

The need for immediate replacement depends on which O2 sensor it is. You have four in the car. Two are pre-cat sensors mounted in the exhaust headers and two are post-cat mount in the exhaust pipe just after the catalytic converters. The pre-cat sensors are a critical part of the engine managment system are their output is used to trim the injector firing for optimal mixture. The post-cat sensors are really just to say whether the catalytic converters are working correctly.

You need to have the fault codes read out of the OBD to find out which O2 sensors are causing the CEL. If it is the pre-cat sensors I'd replace them immediately. Actually, I don't wait for a CEL before replacing pre-cat sensors and will do them when I figure they are getting close to the end of their useful life (80-90K miles). Well before they get bad enough to throw a CEL they are not operating like they should which throws the mixture off. Lazy or weak O2 sensors typically don't cause any noticable symptoms, like a miss or a change in fuel mileage, but the car will be down on power or a bit sluggish when accelerating.

Since the post-cat O2 sensors are "informational only" I only replace them when the OBD is throwing a fault on them.

because your car has given you several warnings that there was a problem and you are too cheap to fix it. Right. :confused. :mad :nono
Fix/maintain your car or buy an accord or camry. It's not the inspectors fault!

The need for immediate replacement depends on which O2 sensor it is. You have four in the car. Two are pre-cat sensors mounted in the exhaust headers and two are post-cat mount in the exhaust pipe just after the catalytic converters. The pre-cat sensors are a critical part of the engine managment system are their output is used to trim the injector firing for optimal mixture. The post-cat sensors are really just to say whether the catalytic converters are working correctly.

You need to have the fault codes read out of the OBD to find out which O2 sensors are causing the CEL. If it is the pre-cat sensors I'd replace them immediately. Actually, I don't wait for a CEL before replacing pre-cat sensors and will do them when I figure they are getting close to the end of their useful life (80-90K miles). Well before they get bad enough to throw a CEL they are not operating like they should which throws the mixture off. Lazy or weak O2 sensors typically don't cause any noticable symptoms, like a miss or a change in fuel mileage, but the car will be down on power or a bit sluggish when accelerating.

Since the post-cat O2 sensors are "informational only" I only replace them when the OBD is throwing a fault on them.

Thank you, very informative.

Interestingly enough, the check engine light went away on my drive home last night, about 12 hours after it came on.

Should I go back to the emmissions place and have the test done, or does the OBC record any recent incidents of the check engine light being tripped? I kinda have this feeling that if I go back they will still tell me I need to change my O2 sensor even though the check engine light went away.

because your car has given you several warnings that there was a problem and you are too cheap to fix it. Right. :confused. :mad :nono
Fix/maintain your car or buy an accord or camry. It's not the inspectors fault!

I referred to the inspectors as 'fools' not simply because they were inspectors but because they were behaving like unprofessional idiots while they should have been working. The emissions testing center seemed like they had recruited all their employees from the local juvenille detention center.

Sorry if I offended you.

Thank you, very informative.

Interestingly enough, the check engine light went away on my drive home last night, about 12 hours after it came on.

Should I go back to the emmissions place and have the test done, or does the OBC record any recent incidents of the check engine light being tripped? I kinda have this feeling that if I go back they will still tell me I need to change my O2 sensor even though the check engine light went away.
The OBD retains certain faults for three "Driving Cycles" at which time they are cleared if the problem has not reocurred. Other faults are retained until cleared with a reset tool. So even with the light off you probably won't pass emmissions inspecstion since they'll see the codes for the previous failure if they haven't been cleared.

The best thing to do would be to have the fault codes read out to see what's been logged as failing. Then fix that and clear the OBD codes. O2 sensors aren't that expensive and are a pretty easy DIY if you have the special tool for removing and replacing the sensor. The sensors are about $100/ea and the tool runs $16-18. While it might be best to replace all four, there's a good chance than only the pre-cat or post-cat sensors are throwing a code and replacing them would suffice for the inspection. Do replace them in pairs, especially the pre-cat sensors.

I just replaced my pre-cat sensors on a 525i and the whole job took less than 20 minutes.

The need for immediate replacement depends on which O2 sensor it is. ...
If it is the pre-cat sensors I'd replace them immediately. Actually, I don't wait for a CEL before replacing pre-cat sensors and will do them when I figure they are getting close to the end of their useful life (80-90K miles)...

Since the post-cat O2 sensors are "informational only" I only replace them when the OBD is throwing a fault on them.

What thejlevie didn't say is if a pre-cat O2 sensor that the ECU uses to monitor fuel mixture fails, the default ECU mode is to richen the mixture.

Eventually, the unburned gasoline will ruin the catalytic convertor. Then you'll be replacing both pre- and post cat O2 sensors and the catalytic convertor, too. Now THAT would be expensive.

What thejlevie didn't say is if a pre-cat O2 sensor that the ECU uses to monitor fuel mixture fails, the default ECU mode is to richen the mixture.

Eventually, the unburned gasoline will ruin the catalytic convertor. Then you'll be replacing both pre- and post cat O2 sensors and the catalytic convertor, too. Now THAT would be expensive.
Yep, that and the excess fuel will coke up the heads and tend to foul out the plugs. The pre-cat O2 sensors are the primary source of information for determining whether the mixture is lean, rich, or just right. So it is important that they always be operating properly.

Hi,
You may want to invest in the Peake Tool that we carry, figure out what is wrong get get it fixed before things get worse.... (and to pass the inspection too!)

Good luck!

If I'm reading this correctly, no one has actually read the OBD on the car in question so it's possible there is something else that needs looking into.

My O2 sensors lasted until 105K miles on a 2000 528.

Reading the OBD II codes is usually the first part of an inspection and it would have revealed the faults in the O2 sensors since they are a part of the emissions system. There could well be other faults, but an O2 sensor fault would be an automatic fail on the inspection.

In every state the inspection reads the OBD codes?

I never had a problem with NY state inspection but when my 02 sensors failed a couple months after, the mechanic told me there were all kinds of fault codes in there that I never knew about becuase I didn't have any problems.

I assumed the faults had been in there for a long time; he said that was typical.