Most garages and many parts houses can load test it for you while in the car.

A slipping belt will cause that message as was pointed out. If that is occuring you may find some glazing showing on the underside of the belt.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)