Help with Front vs Rear O2 Sensors

[ExpensiveTaste]

Hi all, this is my first thread on this forum but I'm active on E90post and N54Tech.

Problem: I'm having trouble getting rid of a code P1093 (Pre-Catalyst Fuel Trim Too Rich Bank 2) that started after getting rid of a P0171/0174.

History: The lean codes were remedied by replacing a torn intake elbow as well as one of the caps on the back of the intake manifold. Both an Indy shop and BMW have smoke tested the vehicle and found no vacuum leaks. At the same time, I replaced spark plugs, air filter, fuel filter/regulator, thermostat, all oxygen sensors, and treated with seafoam. I also swapped all injectors and all coils from bank 2 to bank 1 to see if the code changed to bank 1, but it did not. The last thing I did was changed the exhaust manifold on bank 2 with another junkyard manifold to no avail...

Question: When I replaced the oxygen sensors I got Bosch equivalents (not universal, just not rebranded BMW) and the rear O2 sensors didn't have the same tips as the OEM sensors. The replacement Bosch sensor tips for the rear were exactly the same as the front. Could this mean I got incorrect sensors for the rear?

My thinking: After searching, there are two versions of thought when it comes to the role of the rear oxygen sensors. One version is they are used solely for determining cat efficiency and the other is they have up to 30% determination in fuel trim (which I suspect would be shown in long term fuel trims) on top of determining cat efficiency. The rear O2 sensors are not narrow band sensors and this leads me to believe they are used for fuel trim determination as well. I'm thinking the rear O2 sensor design of the OEM sensors might be slightly different than the front sensors for a reason and maybe Bosch thought they could just swap to a universal and it's throwing off the ECU with slightly different-than-oem readings.

Sorry for the long post, but hopefully that gives you guys enough info as I've been diagnosing this vehicle for the last 2-3k miles. The light will sometimes correct itself (slightly leaner trims) if it's cold out and the car has been sitting all night.

Pictures of the tips for reference

OEM front


OEM rear

[ExpensiveTaste]

Update: I finally installed INPA on my laptop and scanned it. P1093 is equivalent to BMW code 236 which shows up as Oxygen Sensor control upstream bank 2. Any ideas?

[racingbmwm3]

Rear sensors are purely for determing cat efficiency and will not throw any other codes.

Your code is for a pre-cat (front) sensor anyway. If you aren't getting a fault for the rear, then they are fine.

Have you tried swapping the front 02 sensors around to see if the fault follows the sensor? Could be a new defective sensor. Or try putting the original sensor back in place of the bank 2 front sensor that is throwing the fault.

Did you reset the adaptations after doing all that work? If it was running WAY lean before because of the vacuum leaks it could run rich after the repairs. But if you have driven 2k since then, that isn't likely.

So, its not a bad fuel injector or coil, if changing out the bank 2 pre-cat O2 sensor doesn't fix it, then try swapping around the spark plugs. While doing that, look carefully at all the plugs and note any that look different than the others. A black or wet plug is the culprit.

Clean the MAF yet?

[ExpensiveTaste]

Originally Posted by racingbmwm3

Rear sensors are purely for determing cat efficiency and will not throw any other codes.

Your code is for a pre-cat (front) sensor anyway. If you aren't getting a fault for the rear, then they are fine.

Have you tried swapping the front 02 sensors around to see if the fault follows the sensor? Could be a new defective sensor. Or try putting the original sensor back in place of the bank 2 front sensor that is throwing the fault.

Did you reset the adaptations after doing all that work? If it was running WAY lean before because of the vacuum leaks it could run rich after the repairs. But if you have driven 2k since then, that isn't likely.

So, its not a bad fuel injector or coil, if changing out the bank 2 pre-cat O2 sensor doesn't fix it, then try swapping around the spark plugs. While doing that, look carefully at all the plugs and note any that look different than the others. A black or wet plug is the culprit.

Clean the MAF yet?

I did try a variety of things with the pre-cat O2 sensors including 1 old and 1 new in each location, swapping them, etc. I had the adaptations reset by a friend with a special scanner and it took about 300 miles for the code to come back. That was when I decided to try a different manifold on bank 2. The spark plugs were just changed recently and the plugs that came out had no noticeable differences from cylinder to cylinder.

What does this code actually mean? I'm interpreting as the ECU sensing a slight lean condition that is not enough to trigger a lean code, but the computer is trying to richen up the mixture to the point where it hits a max richening value. Is that right?

[racingbmwm3]

No, you have it backwards. The sensors are measuring a rich mixture and are unable to lean it out any more. You need to inspect the new plugs to see which cylinder is having rich running issues. Go from there.