Oxo Sensors keep failing on my beemer!

The “service engine soon” light came on about 6 weeks ago. I took it to the mechanic (tried a new shop recommended by a co-worker; dealer’s too costly!). At first, the mechanic (owner of shop)thought it was the catalytic converter and the 2 front oxygen sensors. He had to order the parts. I took it back for the repair, and was told when I picked up the car it was NOT the catalytic converter, but THREE oxo sensors were replaced – 2 front, 1 rear. All fine for 2 days, then “service engine soon” light came back on. On my 3d visit, mechanic said 1 rear oxy sensor was defective, but replaced not only that one, but the other rear oxo sensor. With all 4 oxo sensors now replaced, I left the shop 2 weeks ago, and the next evening, the “service engine light” came back on AGAIN!! I called mechanic and he said he is mystified. He said “bring it again and we will have a look. You must have a leak somewhere.” He said the odds of getting yet another defective part from his very reliable vendor are slim. I have not yet taken it back - will do so in a few days. I don’t trust this mechanic, but I want to follow through until he gets this right! Any ideas as to what is going on? The car drives fine. Please help!

The problem obviously isn’t with the oxygen sensors, the original ones were probably fine. The new sensors were not defective, his diagnosis was, most likely. I’d have the mechanic put the originals back in and or get all your money back and try another shop.

The service engine light means that there are diagnostic codes stored in the computer. If this mechanic even read them, try to get them (they might even be on an invoice). Or many auto parts stores will read them for free. Get the exact codes (format: P0123) and post them.

As Xebadiah said, you probably never needed them - but lots of people and places replace parts rather than diagnosing problems.

The trouble codes are most likely for a circuit problem. The exact codes would be very helpful. A code reader can give you the codes, but a true mechanic would have a scan tool that can get the readings from every sensor on the car. With the engine running, a scan tool can show the mechanic the readings the computer is getting, and determine if the sensor is bad or maybe the circuit that connects to sensor to the computer.

It is theoretically possible to get a bad new sensor, but unlikely.

If this is a post 1996 (OBDII) BMW, the sensors are due for replacement at around 100k miles, so if you have that many miles, you have not lost much in having the sensors replaced, even if that is not the only problem. It sounds, however, like you have purchased a total of 5 sensors, so that sounds fishy.

The next thing to check is whether the heaters in the sensors are all getting power. I had intermittent power to an oxygen sensor heater once. Worked when I checked it but failed while driving. Drove me crazy until I discovered the problem.

Thanks to all who replied. My car has only 78k miles (sedan). I bought it new in August 2002. Yes, the mechanics ran the diagnostics, but I don’t have the codes with me. They are at home. First I went to Auto Zone for car light bulbs, and they ran a test for free - plugging their hand-held scanner into the plug under my dash, with the car running. Their codes said something like “secondary air system.” I have the codes at home and can supply them in a day or two. The mechanics got the same codes when they ran their scanner.
After first replacing 3 sensors (2 in front, 1 in rear) - for which I paid - the mechanic did the subsequent labor for free- replacing the 1 he had installed in the rear, which he said was defective, and replacing the other rear sensor as well. So - while altogether, 5 have been installed, I have paid for 3. I’m sure the originals are long gone. I just called the shop manager and asked if he had ever heard of an issue with the “heaters in the sensors getting power.” He hasn’t, but he’s not the real mechanic. The shop owner is a real mechanic, but is on vacation for a few weeks. His other mechanics are available to take a look. After paying this shop all this money, I want to make them get it right! But I won’t go back to this shop ever again after this gets sorted out. (Although - after one more try, you might be right that I should just move on - but I can’t imagine them giving me my money back!)

I just called the shop manager and asked if he had ever heard of an issue with the “heaters in the sensors getting power.” He hasn’t, but he’s not the real mechanic. The shop owner is a real mechanic, but is on vacation for a few weeks.

That is a bad sign. The codes will be needed to provide any real feedback, but I can say that “it just needs a new oxygen sensor” is something of a legendary response to lots of different codes. I once asked a guy at an auto parts store to pull my codes for me. He looked at me and said “Why? Its gotta be an oxygen sensor. That’s all it could be” - holy shhhimoke! (It turned out to be an evap system code.)

Anyway…if the codes are about the secondary air system this is a system that pumps extra atmospheric air to the catalytic converter to help it burn better. The O2 sensors are wrapped up in these systems since they basically measure gases before and after the converter. I think it is likely that your shop fell for one of the classic myths - that if an O2 sensor reports a problem than the sensor is the problem. Meanwhile, many a competent and hard working O2 sensor gets tossed aside simply for doing its job in the right way.

There various codes for the secondary air system and each can point in different directions. So do post them when you get home and maybe someone can give you some specific advice.

Hi there,

Your mechanic, while cheaper than the dealer, has already cost you way more money, and doesn’t know how to properly diagnose your car, which in the long run, is going to cost you far more money.

Since I don’t know your car very well, and you didn’t include the year in any of your posts, I will have to go off of my knowledge from experience with my Porsche Boxster, and the way its system works with regards to the check engine light, and O2 sensor messages.

With the Boxster, whenever I see the CEL come on, and it shows that both banks of the O2 sensors are reading out of range, and the computer can not adjust the fuel trim any more to keep the car within specs, I start by diagnosing the MAF (mass air flow) sensor.

Why do I do this?
Pretty simple:

It would be a rarity for all 4 O2 sensors to fail at the exact same moment.
So, if you are getting an error code that says that both banks can’t be properly balanced, you need to look for a part that controls the amount of Oxygen that the engine gets, that is common for all cylinders.

In this case, this rules out the individual fuel injectors, because that would typically just cause a misfire on a specific cylinder, or would cause 1 bank of cylinders to reach the rich or lean limit, depending on what way the injector failed.

This also rules out the individual spark plugs and ignition coil sticks for similar reasons.

Basically, you are then left with the throttle body, and the MAF sensor, as these report how much air is being let into the engine at a given throttle position.

If the MAF sensor readings are out of spec, then the computer will think that the engine is getting more or less Oxygen than it actually is. The computer then increases or decreases the amount of fuel that the engine uses, which then after being ignited, passes the O2 sensors, and the computer checks to see if its running lean or rich.

So, if the MAF sensor fails, your computer will think that the O2 sensors are out of spec, and will set off a CEL saying that the O2 sensors are the issue, when it clearly might not be the case.

If your mechanic has a decent diagnostic tool that can read the MAF sensor readings, there is most likely a test procedure that compares the MAF readings at idle, and at a higher engine speed. The MAF readings should then be between a specific range at those two rpm points.

Good Luck,

I had said in one of my replies that my car is a 2002 - I bought it new - and it now has about 78,000 miles on it.

I will still put the codes out there tomorrow for all of you folks to consider.

The diagnostic codes that showed up on the scanner were:
P0491 – Secondary Air System (Bank 1)
P0942 – Secondary Air System (Bank 2)

Those were the codes that I got first at the auto parts store, and then when the mechanic ran his diagnostics, he got the same codes.
He then replaced all 4 oxy sensors (albeit for the price of 3).
Interestingly, yesterday I went back to the auto parts store b/c yet another bulb-message came on my dash. I asked him to plug in his scanner again to try to see why the “Service Engine Soon” light is back on again.
Yesterday, he got those exact same codes - P0491 and P0492.
By the way, do you all think it is just coincidence that within the past month or so, all my various bulbs are needing to be replaced?? First it was the regular break light on one side and the top mount break light, then a week later it was the back-up light on one side, then it was the “halo light” on the right side front, then within a week the license plate light (one) went out, and now my lowbeam light bulb signal is on the dash (which is why I went to the auto parts store again yesterday).
It seems a little wierd after all these 7 years and 4 months, all these various bulbs would be failing in close proximity to one another.
It’s a secondary concern for me right now - compared to the “service engine light” issue - UNLESS you all think these problems could be related to one another.
Many thanks!

Find another mechanic. All four O2 sensors should not have been the first thing he decided to do for these trouble codes. As mentioned earlier, these codes are for the secondary air system, which is designed to pump air into the catalytic converters to improve their efficiency. This system usually has a belt-driven air pump, a valve box, a check valve or two, and hoses and pipes. My generic code list doesn’t have a reading for P0491 and P0492, so specifically what is wrong with the system I cannot pinpoint. However, the most common problems are carbon clogging of the injection pipe at the cat or the check valve. Failure of the valve box and pump can also happen.

You need to find a mechanic that specializes in BMW, some one other that the mechanic who sold you 4 O2 sensors that probably didn’t need replacing.

I certainly think that replacing the O2 sensors in response to this was nuts - unless there is some rationale the mechanics had that hasn’t been conveyed.

As I understand it, problems with these systems have been common/chronic and notoriously difficult to diagnose.

There are people on these boards who might have specific knowledge/tips. If you are determined to track this down yourself I’d suggest a thorough combing of BMW dedicated boards - here is one thread on the issue: http://forums.bimmerforums.com/forum/showthread.php?p=16031339 - that eventually points to a somewhat hidden 50A fuse in the secondary air system. (Bonus at top of thread is a diagram - might not match yours specifically, but the basics will likely be similar).

There also could be tech svc bulletins on it (TSBs).

Other than that, this is the kind of thing I would take to a shop that specializes in BMW (not a dealer). And before I had them touch it I’d have really long conversations about their experience with the system and its problems. If you do search the issues for a while you’ll find lots and lots of blind parts replacement without remedy. So you really want someone who is serious about diagnosis - rather than throwing parts at things.

Also, it is not uncommon for light bulbs to start going out enmass like that. They are generally designed to last the same amount of time.

The mechanic you used should never have replaced the O2 sensors after pulling those two codes from your car.

After a couple quick searches on Google, there appear to be a couple quick checks you can do on your car to determine what might be the problem.

First off, you should hear the air pump come on after you first start the car for about 30 to 90 seconds. If you don’t hear it come on, the pump might be bad, a relay might be bad, or a fuse might be blown.

If it does come on, the next step is to inspect the check valve.

To check the valve, pull off the hose that connects it to the air pump. Reach in to the valve and run your finger around the inside of the fitting. If your finger comes back black with soot, the valve has failed and will kill your air pump (if it hasn’t already). Never replace the pump without replacing the valve too - the valve failing is what causes the pump to die.

To test if the pump itself is working, pull the hose off the pump and started the car cold to see if air was blowing out. No air = bad air pump.

That’s what your mechanic should have done.
You should probably take your car to a BMW dealer and have them properly diagnose the issue for you if you can’t do it yourself, and then you can have your mechanic do the actual work, if you can’t do it yourself.


To be honest, from the sound of it I don’t think he’s ever going to get it right unless he just happens to luck out by replacing or repairing something inadvertently.
O2 sensors have to be one of the biggest automotive scapegoats on Earth.

A problem in the secondary air system could cause readings that may point to an O2 problem but (and just my opinion here) is that the first step that should be performed when diagnosing an engine performance problem is run a vacuum check for any leaks.
A vaccum leak can cause all kinds of grief (including O2 codes) and considering that this test can be done in 30 seconds with in inexpensive vacuum gauge it’s best to weed that out right off the bat.

Thanks all. I do feel like I made such a bad choice with this mechanic! He came highly recommended - but now I know better.
I should say that, after first replacing 3 of the 4 oxo sensors, the “service engine soon” light DID GO OUT. It was off when I picked up the car and stayed off for at least 24 hours (can’t remember exactly).
When the mechanic subsequently replaced what he determined was a defective oxo sensor in the rear, plus the other rear oxo sensor, again, the “Service Engine Soon” light DID GO OUT again - and stayed off for more than 24 hours (but not 48).
That doesn’t make sense to me that it would go out - as if the problem had been fixed - but then came on again.
I have absolutely no way of checking any of those valve things myself - I’m NOT car-savvy at all - but I have learned a lot from all of you. (Yes, I can read the manual and change bulbs, air filters, and easy things - but beyond that, no way.)
I recently got the name of another service repair shop and the name of the specific mechanic there who specializes in BMW’s - and at the advice of one of you, I am going to have a long long chat with him and share all of this info that all of you have provided - before I let him touch the car.
Meanwhile, how urgent do you think this is? Is it safe to still drive the car? Am I doing some major damage by continuing to drive with this light on the dash???
Many thanks!

It is likely that the mechanics reset the light each time. That is standard practice. So the light would have stayed off until the computer picked up the issue again.

If the light is not flashing then there is no emergency and you likely won’t even notice anything.

But if you let it go (as in just ignoring it long term) you would run the risk of creating bigger problems - as in, if one part of the system isn’t working correctly it can harm other parts of the system - in this case the system also includes the cat converter. All the while you’ll be producing more emissions that the car was designed to produce.

So I’d say, drive without worry, but have it fixed as soon as you can.

Many thanks! I’ll get to it in the next few weeks for sure.

The Secondary Air System MIGHT affect the oxygen sensors. It pumps air into the exhaust manifold. The unburned exhaust gases can, then, use the additional oxygen in that air, to further oxidise (burn) any unburned fuel. The oxygen sensors, which are downstream, take readings of the exhaust gases. The emission system is designed to incorporate oxygen sensor readings from exhaust gases which have the injected air. If the air is not injected, by the secondary air pump, it screws up the air/fuel management by the engine computer. An experience repairer on vehicles which have secondary air pumps (on some years of Chevy Ventures, etc.) can tell you of the negative effects on oxygen sensors and catalytic converters.

The Secondary Air Injection Pump, on your 2002 BMW, is probably NOT a full-time air pump. The repair manual would clarify this point. It, like Chevy and GMC, operates during the period the engine is cold; then, shuts off.
So, the problem with the oxygen sensors should be troubleshoot separately.

Sorry, but I did not read your original post carefully enough. As I recall from my wife’s e46 Owner’s manual, they moved the response to a loose gas cap from the check engine (CEL) light to the Service Engine Soon light. FYI, the other thing it said on that page, as I recall, was that a flashing Service Engine Soon light means very low oil level. Any code related to oxygen sensors will illuminate the CEL light.

If the secondary air pump is failing, it will usually make a terrible metallic churning sound when it runs for a couple of weeks. If you start the car cold with the hood up, you can hear the secondary air pump running for the first minute or so. It sounds like a vacuum cleaner.

Come to think of it, I noticed that my wife’s 2004 with 80k miles on it was sounding pretty nasty when she started it the other day. I did not think much of it at the time, but now I am thinking that it sounded like her secondary air pump getting noisy. This is about the mileage where she lost the pump on her '97 328 a few years ago. Darn.