Pre-Cat O2 Sensor Location?

[jffspicoli]

Apologies in advance for such a stupid question, but I have a 2004 4.4 E53 and need to replace my pre-cat o2 sensors.

I've read that on some X5s they're under the car and on others they're in the engine compartment. I've looked under the hood and can't find them.

Can anyone confirm the location?

[jdstrickland]

You are looking for Items 5 & 6, here

RealOEM.com * BMW E53 X5 4.4i CATALYST/LAMBDA PROBE

[jffspicoli]

Thank you for your help.

Just curious, so then is it true that some have them under the car and others in the engine compartment?

[jdstrickland]

The location of the O2 sensors has to do with the space available. The function of them demands they be upstream from the CAT. There is another set of sensors downstream from the CAT. There are some applications where there is another CAT, and if this happens then there is another set of sensors after that CAT.

So, it is true that some have the upstream sensors in the engine bay and some have them under the car. But they must be upstream of the CAT, being in the engine bay or under the car does not matter, and is determined by the space available. There is an O2 Sensor on both exhaust banks. (4 cylinders seldom have more than one bank, but they could, and if they do then there are two sensors.) Your V8, and all BMW inline 6s have two banks, so there are two sensors -- one upstream from each CAT.

There is another sensor down stream of each CAT, and if additional CATs, then a sensor will be found after each of them.

The way to find the sensors is to first find the CATs, then look upstream (towards the engine as the exhaust flows) to find the sensors that are Items 5 & 6 in the diagram I gave you, or downstream to find the sensors that are Items 7 & 8. CATs are always as near as practical to the engine, and well before any mufflers that are present. Follow the exhaust out of the engine, the CATs will be the first component that looks like a can. The O2 sensors before the can are tasked with monitoring the contents of the exhaust, the O2 sensors after the can are tasked with making sure the exhaust comes out of the can cleaner than when it went in.

Why are you considering replacing the O2 Sensors?

[upallnight]

On a X5 3.0 M54 engine the pre-cat sensors are in the engine bay and can easily be removed from above.

[jffspicoli]

Thanks for the detailed explanation. It sounds like the pre-cat o2 sensor location may depend on the year/model of X5s? Since I couldn't find them under the hood, I'll crawl under the car again.

I'm replacing the pre-cat o2 sensors because I got a fault code after leaving my car dormant for months. It has about 100k miles.

Appreciate any advice in advance.

[jdstrickland]

What code did you get?

I think that on the V8, you cannot get to the sensors from the top. Follow the manifold out of the engine, there is a coupling where the manifold bolts to the CAT. The sensors you are looking for are an inch or so after this couple. There is too much stuff in the way on the V8 to see the sensors from the top. I have the 3.0L, and the sensors can be changed from the top.

[jffspicoli]

Just checked my Peake tool and I'm no longer getting the pre-cat o2 sensor code, but now I'm getting a different code - 2772 "Control Tank-Ventilation Valve".

I drove this car less than 1k miles over the past year, so I guess I shouldn't be surprised by the random fault codes. I also found mouse piss/poop on the plenum cover, so now I have mice living in the engine compartment. wtf.

[jdstrickland]

Get an ELM-327 module that plugs into the OBD II Data Port under the dash. Then get an app for your phone or tablet -- android or apple -- and use it to collect the data. The app generally has a free version, Torque Lite, and a fee version, Torque Pro. the fee version only costs 5 bucks, so you can collect real time data for less than $30, certainly less than $40. You get better intel about the inner workings of your car than Peake will give, and the codes are in a standard format, Pnnnn, that we know. You can take the module from car to car and read codes, collect realtime data, and reset the codes. You can also look into the PA Soft 1.4 if you want to go into the inner workings of your car and change some of the settings.

Personally, I'd be driving this car and pulling codes if the Check Engine light comes on, then make a list and reset the codes and keep driving. When the light comes on, see if the current codes match any of them on the list that you are building. Fix the repeats, ignore the unique codes.

[jdstrickland]

You get the module on eBay or Amazon. They have them in wi-fi and bluetooth versions. The expensive ones have both.