I've had trouble with my EGR's  They seam to go bad w/o tripping the sensors.  I get symptoms, but no light.  The first time it happened, I guessed at it and got it right the first time.  This hasn't happened in about 2 years.  Can they be partially clogged enough to cause symptoms, but not be totally sensor tripping? Aren't the O2's the tripper sensors for that? Sometimes it will: stall w/o killing the car or idle @ low RPM.  I replaced the EGR the first time... and problem fixed.