"He said they were initially put on to control emissions, but
he has tested buses before and after installing new sensors and
couldn't notice any change."

Marc,

The sensor is used by the computer to help set the optimum mixture for low 
emissions.  The problem is that the sensor has a finite life, but cannot be 
tested except by replacement.  The idea is that for a particular vehicle, 
the sensor is supposed to last at least some number of miles.  This is the 
mileage that the warning light comes on at.  The new sensor is not supposed 
to improve the way the bus runs, but rather prevent the bus from running 
badly at some future time.

How long will that be?  It's a guess.

Will you notice a bad sensor?  Perhaps.  If you live in an emissions test 
area, yes.

Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com