BMW Repair: Idle Speed Control Valve, coolant temperature, engine temp

Question
I have a 1997 740iL (E38) (VDL44763).  The check engine light is on and the fault code I extracted was P1550.  I thought it might be the valve that was bad, but I ruled that out with a new one.  How do I check the curcuit, and what else should I be looking at.  The car idles fine although it doesn't increase the idle speed when it is cold.  Any sugestions.

Answer
Darren, use your OBDII to look at coolant temperature status and make sure it is correct and changes with actual engine temp. You can remove and inspect the sensor too to make sure it is not full of calcium or other build up. Check for intake manifold leaks with a smoke machine or favorite other method. Mass airflow is always possibility, but it doesn't sound like that is what you are up against.
Sorry I'm not a great deal of help on this one. Let me know how it goes.
-Evan