2005 BMW X5 DME P1093 Code
This 2005 BMW X5 was brought to us as a project car. It had a P1093 pre cat fuel trim too rich, bank 2 code that no one seemed to be able to cure. All 4 oxygen sensors had been replaced. The oxygen sensors had also been swapped bank to bank. Spark plugs had been replaced. Coils had been swapped between banks. The air mass meter had been replaced. The injector seals had been replaced. And, a few other things had been done that I can’t remember.
We started by confirming the code. The P1093 code was set and would reset after 2 drive cycles predictably. Examining the freeze frame data and the live data showed everything was well within its limits except the rear oxygen sensor values. They seemed read in opposite directions, but since the code was for a pre-cat issue, we did not see how those readings could trigger a pre-cat code. We also smoked the intake system. We followed the diagnostic plan in the BMW factory scan toll ISTA, and that found no faults. Since everything else seemed to be functioning normally, we started to believe that the DME may be falsely reporting, so we found a used one, had it cloned for the vehicle and took it for a drive. This had no effect. 2 drive cycles later, the light came back on.
So now we had a real mystery to solve. I started going over the vehicle item by item. I found a small cut in one of the injector seals. I though “YES”, this must be it, but how sensitive this system is. 2 drive cycles later the vehicle said “NO”, that’s not it. I hooked a fuel pressure gauge and went driving up the highway. On long accelerations, I found the fuel pressure would drop. I removed the fuel filter and you could not even blow through it. Again I thought I found it. A fresh fuel filter a few test drives, and we would be good to go. This vehicle did not agree. 2 more drive cycles and the service engine soon light came back on. I knew the air mass had been replaced, but I have seen them do some strange things, so I put another on to see if there was an affect. There was none. 2 more cycles, and you get the idea.
Now, one thing that kept nagging at us was the rear oxygen sensor readings one was going rich, and the other going lean. While we could not find any information that the P1093 pre cat fuel trim too rich, bank 2 code was in any way affected by the rear oxygen sensor readings, I needed to explore this a little further. I theorized that the converters had failed and that was preventing the DME from being able to test the oxygen sensors. The DME would then trigger the P1093 as a result of this. Since converters are over $2,000 for aftermarket units, I need to come up with a way to test my theory. So I installed 2 rheostats to control the rear oxygen sensor values.
I took the X5 for several test drives. Each time keeping the rear oxygen sensor values in the mid-range. The SES light never came back on and the P1093 code never returned. The customer was happy that we were able to solve this problem that he had been fighting with for years. He gladly had us install 2 new converters and is enjoying his X5 free of the service engine soon light.
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