BMW: Oxygen Sensor, 1995 bmw 740il, bmw 740il

Question
 I have a 1995 BMW 740il. When it gets cold, the engine runs very rough and has poor idling. The engine check light also comes on. We recently had it checked out by a mechanic and the OBD tests showed that there was a problem with the oxygen sensors and recommended that we replace them. When the code is cleared, the car runs well. Do you think that this is a problem with the oxygen sensors or could it be something else? It only happens in extremely cold weather (freezing or near freezing).

Answer
David,
 Take a look at your temp gauge when the car is (or should be) up to temp.  If the car is running on the cool side (i.e. it never fully warms up), you might have a stuck/broken thermostat.  The coolant sensors will tell the computer that the engine still in "cold" mode and it will override the O2 sensor.  If this happens for too long, a fault code will be produced.  
  I would replace the Thermostat if it has not been done in the last couple years.  Thermostats go through MANY heat cycles, and this is like bending a paper clip back and fourth.... eventually it will break.

 I have a 99' 328is that this happened with.  It was throwing a fault code for O2 sensors, but this only happened when it got really cold outside.  I then noticed that the engine was running pretty cool and that's not good.  So I checked the thermostat and sure enough it was broken.  After I replaced it, the Check Engine light stopped coming on.

 O2 sensors only usually have a life of ~50k miles.  So it could be time to replace them anyways.  So if you're engine is running at normal temp (usually right in the middle) then you probably need to replace the 02 sensors (I think there is 4 of them, 2 on each side)

 Hope this helps,
 Josh