1993 850 Runs very badly with lambda/HO2S connected
 

Hello,
I am having troubles with my 1993 850 2,5 125kW. The problem is that whenever the HO2S is connected, the engine starts to run worse and worse over the course of 10-20 miles, to the point where it won't even rev up in neutral, and has no idle. After a while, the OBDI A2 socket gives out 511, which should stand for Long Term Fuel Trim Idling, Lower Limit, which, as I understand, means that the fuel mixture was too rich and the ECU had tried to make it leaner as much as possible, but had hit the lower limit. When the DTC is cleared, the engine runs well for about 10-20 miles, and starts stuttering again.
I first suspected the MAF-sensor, but replacing it with one from an identical car didn't make any difference. Then I discovered that when the HO2S is disconnected, the car will run rich and take a lot of fuel, but the engine runs more or less smoothly.
Now the problem is, after replacing the HO2S with another one, I am still getting the same issue when its connected, forcing me to still drive without it.
What else should I look into, that could cause problems while the HO2S is connected? I noticed that the exhaust gas is quite hot both with and without the H2OS, and I was suggested that the cat. converter could be busted. Could that be the cause of my problems, although it is behind the HO2S in the exhaust piping?
Steven.