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Discussion Starter · #1 ·
I have a 1997 BMW 540i and it failed the emissions test in Mary.....land. I believe the car is fine except for the fact that the sensors that test the exhaust are not working. The state refuses to actually test my exhaust. They just plug a wire into the computer and it tells them it fails.

I am wondering what these sensors are, and how i can get them? And once I get them will replacing them solve the problem. Also, how do I install them?

Thanks in advance.
 

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You'll need to go to a good independent BMW shop. They can read the stored codes and determine what's wrong. It could be bad O2 sensors in the exhaust, or bad catalytic converters.

There are four sensors in the exhaust system, two for pre-cat, and two for post-cat.

Most V8 BMWs with 100k+ need new cats.
 

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I believe the sensors you are referring to are the oxygen sensors. You will only need to replace the two pre-cat O2 sensors as these play a critical role in emissions control. Reading the codes is a good idea so that you don't throw money at parts randomly. To remove, cut the sensor wire and remove with a deep socket. Unplug the wire up near the valve cover (that's where it is on the I6). Screw the new O2 sensor back in. You'll need an O2 sensor socket (one with a split down the side to let the wire out) to allow you to tighten the new one. Reconnect the wires and you're done. It may take a few miles for your computer to register the new sensor.
 

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Discussion Starter · #4 ·
Thanks but I am clueless, are the pre cat sensors the ones that tell the computer if everything is working? The stupid laws do not allow them to test the exhaust, they only go by what the computer says.

As for installing them , I am not even sure where they are.
 

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Thanks but I am clueless, are the pre cat sensors the ones that tell the computer if everything is working? The stupid laws do not allow them to test the exhaust, they only go by what the computer says.

As for installing them , I am not even sure where they are.
Probably time to simply take your car to a qualified independent BMW specialist and "pay the piper" to get it done...

In the simplest of steps you need to do the following:

1) Get the codes read to find out what exactly is wrong with your car. This requires an OBDII code reader, but most any autoparts store will read the codes for FREE.

2) Post the codes in this thread, and we can assist you in what your next course of action would be.

Based on your first post, it would appear the emissions testing facility believes the codes are related to possibly bad O2 sensors. As previously stated, your car has four of them, and the codes can determine exactly which of the four are bad, and need replacing.

Does your car have the "service engine soon" light illuminated? It should. If not, you need to fix that. And if so, why have you been driving with it on, and not take it in to get checked? ;)
 

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u mean 02 sensors? they are easy to replace..u can always buy the universal o2 sensors and rewire them by yourself..i think bosch makes universal 4 wires 02 sensors for like $75 at your local auto parts store..but u need to determine which one went back because i think there are 4 of them on e39
 

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Not sure about the US but in the UK 1997 540's only have 2 sensors (pre-cat). The post-cat sensors don't appear until 98/99
 

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Discussion Starter · #9 ·
Thanks guys......the code that shows up for oxygen sensors is P0133. Is there a list of codes somewhere that anyone could show me?

As for why I am still driving it, I was laid off last year when our healthcare premiums went up. The car seems fine but Maryland will not test the exhaust, they only test the computer. I got a carfax report before buying, not worth the paper its printed on.
 

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Thanks guys......the code that shows up for oxygen sensors is P0133. Is there a list of codes somewhere that anyone could show me?

As for why I am still driving it, I was laid off last year when our healthcare premiums went up. The car seems fine but Maryland will not test the exhaust, they only test the computer. I got a carfax report before buying, not worth the paper its printed on.
List of codes somewhere? Yes, it is called the internet! ;) I used google: http://www.obd-codes.com/p0133

You need to replace the pre-cat O2 sensor on bank 1 (passenger side). Don't keep driving it that way, you could damage the catalytic converter, and that is really expensive.

If you have a jack and jack stands you can replace the front O2 sensor in about 15 minutes.

I see them online for around $88 with free shipping for a factory Bosch one. BTW, get the assembled one with the plug already on it. Do not try to save money and get the "universal" one that requires you to splice the old plug on your new O2 sensor...to much possibility of messing that up. ;) Only buy Bosch brand, again, to eliminate the chance of a incompatibility issue. BMW only uses Bosch, you should too. I would replace them both, as the other side could go in the next few weeks or months. But if money is tight, then just do the one bad one. Good luck and report back. After you replace the O2 sensor, you will need to go back to have the light/code cleared. It will go out on it's own, but could take many, many drive cycles for that to happen.
 

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u mean 02 sensors? they are easy to replace..u can always buy the universal o2 sensors and rewire them by yourself..i think bosch makes universal 4 wires 02 sensors for like $75 at your local auto parts store..but u need to determine which one went back because i think there are 4 of them on e39
Bad idea. Non-OEM sensors are generally a bad idea in these cars and then the manual wiring job on sensors that work with small differences in resistance... no.

OP: It doesn't matter what the actual exhaust smells like or is testing-the car is unaware of how efficiently it is operating and has resorted to default best-guess, which increases fuel consumption, emissions, and will accelerate the wear of your catalytic converters, which are very expensive.
You do not NEED to replace all 4 at once but chances are very high that the other pre-cat will fail soon; replacing separately doesn't necessarily mean extra labour but it does mean extra trips to the mechanic's. The post-cat O2s very seldom fail***8212;they're the ones that monitor the efficiency of the catalytic convertors. You will need to replace all of them if all of them are throwing codes.
Post all the codes you get and look them up with Google.
 

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thanks it will be a few weeks at least. one more question though, are all 4 sensors the same, I need to replace all 4 due to other error codes that came up.

Thanks again.
Per what was said in post #14 above, you only need to replace the O2 sensors for which you have codes for. IF you have codes for the post-cat O2 sensors, then yes go ahead and replace them as well, or you will continue to get codes and the light for that problem. Different part number, as they have different length cables, and possibly a slightly different response curve.

Just use the look up at the online seller of your choice to order them.

Again, as stated in post #14, the post cat O2 sensors rarely fail, and their only function is to monitor catalytic converter performance, they have no bearing on engine performance. The pre-cat O2 sensors are involved in the feedback loop for engine management, and directly impact engine performance.
 
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