Its been a while since the first post of my problems. I have replaced the Air Flow Sensor with admittedly a second hand one which I was confidently told was working perfectly. It certainly looked far newer than the one it replaced. After fitting the engine performed almost as good as new, the stalling and low power were immediately banished. However the idle while much improved still was a bit lumpy when in Drive with revs below 600rpm. In Neutral revs were around 600-700rpm and much smoother. From cold the idle was smooth and only started to hesitate when hot. For much of the time there is no problem and then it goes into a rough patch with some surging due to the engine management increasing the revs to prevent a stall.

I have done some checking of the codes with the LED device as this is what I have found.
Pin 8
Code 4 Air mass sensor
Code 13 OS2 control system too rich or lean

Does this mean that both the AMS and the O2 sensor are at fault or can the O2 sensor cause the Code 4?

When I cancel the codes and take the car for a run I only get the Code 13. But after a longer run or a few days I get the Code 4 come up as well.

I am reluctant to replace the AMS if the problem is with the O2 Sensor. Also does the O2 sensor cause the Code 13?

As always your help is much appreciated.

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1993 320E 140,000km