Auto Parts: Low idle Rpm, gm cars, crank sensor

Question
I recently had a problem with my vehicle.  It cut off, I cranked it, and it cut right back off.  It did this a few times before I called to have it towed, but while waiting, I tried it again, and it seemed to work fine.  Diagnostic test came back clean.  2 days later, after getting some gas, I tried to crank it and it wouldn't turn over.  After 10 minutes, it finally cranked then the 'service engine soon' light came on.  Turns out, the crank sensor was going bad in it.  I had that replaced.  However one thing I noticed in that few days was that my rpm seemed really low.  Of course I figured it was part of the problem I was already experiencing.  It idled below 1 (halfway betw 0 and 1) when I was stopped at a light.  Since getting the sensor replaced, it's still doing that.  I never noticed where it idled before that whole thing started, so is this normal?  I'm paranoid it's going to start shutting off again!

Answer
    Idling at 500 RPM is not super low, but most GM cars are programmed to idle around 800 RPM.  You may have, in addition to the bad crank sensor, a failing idle speed motor (the gizmo that actually controls the idle).  It would be a good idea to have someone check the computer with a proper OBDII diagnostic device and see if the RPM reading on the dash (which comes from the computer) matches the actual idle speed.  Then, while they are hooked up, they can run a cycle test on the idle speed motor and see if moves through the full range of motion.  It would be good if this test could be started on a cold engine, which would mean leaving the car at the shop overnight.