BMW: BMW 1993 525 Emissions, emissions controls, emission problems

Question
I am currently have emission problems. The NO2 levels are high. I have replaced the muffler, catalytic converter, O2 sensor, and maintain regular regular oil changes. I have failed emissions twice.....HELP PLEASE.

Answer
I am not familiar with the 5-series as much as the 3-series, but it is disturbing that you have changed all of the stated components and are still failing emissions! If you have changed all these parts, which constitute most of the emissions controls, you should be good.  Check your Thermo-time switch, this sensor tells the ECU that the car is up to temperature, and will negate the cold start function.  In most cases the ECU still thinks the car is cold, because this sensor is malfunctioning.  When this happens the car runs rich (too high fuel:air ratio) and contains high levels of fuel in the exhaust.  This will lead to poor emissions quality and possibly why you are failing.  Over all, the car probably could use a tune up if not done in the last couple years.  I dont remember when BMW stopped using the M20 in the E34 5-series, so you could have 1 of 2 possible engines.  The M20 (old 2.5L) or the M50 (Newer DOHC 2.5L).  I am guessing you have the M50, with the black plastic valve cover, but I could be wrong.
 If you have the M50, you should have "coil-on-plug" ignition, where you have 6 individual coils (1 per plug).  I would look at a tune up kit, air filter, new spark plugs, plug wires etc. (if applicable).  This should make the engine run better even if it doesn't help emissions.  
 Also check to see if there are any "fault codes" present on the system.  If the "check engine" light is on, then there is definately a fault present, and could be manythings.  Have a trusted mechanic put his scan-tool to your car and check for fault codes.  This will give you some clue as to the status of the sensors and so fourth.  
 Hope this helps,
 Josh