Pre-Cat Oxygen sensor?
 

The ones that are changed when something is amiss, which are they?

Try:

http://www.lambdasensor.com/main/bmw1.htm

Also knowing year and engine is useful...

Mine's a '08 3.0si. The webpage you posted doesn't list E70. Also it says front/rear but which is the pre-cat?

The diagram above is specific to my vehicle as it's from the realoem site, I put in all the info before going into detailed parts.

From the direction of the arrow in the diagram, I would guess that #1 and 2 are the pre-cat. The pre-cat sensor should be more in front and the post-cat sensors should be behind them. Have you had the codes read? They should tell you which "bank" and whether they are pre or post-cat.

With my engine light on, I checked and am getting some codes in my INPA tool for my 2008 E70 3.0Si:

2C3E A1 Lambda probe in front of catalytic converter 2, control failure or cable transmission failure?
2CA7 A1 Lambda probe heating in front of catalytic converter 2
2C9D A4 output heater O2-sensor before catalyst bank2

So, definitely looks to be pre-cat Lamdba. I'm a pretty good DIY'er, but haven't replaced a probe yet. I read that the pre-cat ones are easier. Any links to good pics/description of what's involved would be helpful if anyone has them. How can I tell which one is bank 2?

I can't tell from this diagram if it's number 1 or 2 that I need.

I found both parts (Bosch) on Amazon. Part 1 is a generic description, where Part 2 has in the description name "Oxygen Sender PRE-Cat Rear Manifold".

I would strongly suggest you find a WIRING DIAGRAM for your o2 sensor sand check EVERY fuse and connection....

When a car is started cold, the O2 sensors do not work- the car begins to warm up, AND the DME sends current to the O2 sensors (heater wihin the sensor) to get them up to temperature. If that circuit is damaged, you get an immediate "o2 heater circuit failure'. Each O2 sensor has 4 wires- one pair is hte acutal signal, the other pair is the heater circuit.

So check the connections first.

if you have 50k miles on them, you can go ahead and replace them- but if you just replace them and the error is there, you'll know what it is.

Also DO NOT wasted you money on OE BMW. Bosch OEM is perfect~$100.

Finally, the most important question: which bank is which? dunno. Sorry. Id replace both so it wouldnt matter to me.

Revising an old thread that I commented on. I let my engine light stay on for a while and finally took it in for other service to an indy and had him verify the codes. He found that the 2 "front" O2 sensors need replaced. His quote was $750 all in. Seems high, considering the parts are about $130-$150 each?

Based on the realoem diagram
RealOEM.com * BMW E70 X5 3.0si Lambda Probe Fixings

It looks like I need number 1 and number 2. But I'm still trying to find a good DIY with pics for a 2008 E70 3.0. Looking at my engine, it looks like there's a fair amount of stuff to remove first to get to both of them, but some guidance from anyone who's done it before would help.

What is the mileage of your X5?It is very rare both sensors to be bad,if not to say that it never happens.If,for example, there is a lean condition due to a vacuum leak,the ECU may throw a O2 sensor related code,but there is a difference between code pointing to the 02 sensor and bad 02 sensor.In the original BMW programs there are tests to be run to evaluate the condition of the sensors. If the Indy wants to replace both the technician has to show you the reason to be replaced or take responsibility if after spending $800 the problem persists.

almost 70,000 miles on it. I haven't picked the car back up yet. I agreed to some other work I needed done, so I'll ask him to elaborate more on how he determined that 2 needed replaced.

He told me both "front O2 sensors". I presume that to mean both "pre-cat" (though I'll double check this too), which if I understand right would be both on my left as I'm looking down at the engine. The one closest to the front I can see. I can't easily see the one further back towards the firewall, which makes me ask how much effort is involved moving things to get to it if I should do this myself?

+1

:iagree:

The sad truth is, however, that after he spends $750, and the problem comes back, then there will be ANOTHER issue, that "most likely is what caused the sensors to fail" for more $$$. Nobody EVER gives back cash for the poor diagnoses they perform (present company excluded. ;) )