GMC 2003 DURAMAX Owner's Manual page 18

ELEVATED IDLE ENABLED
On diesel engines, this message will appear when the
elevated idle/exhaust restrictor feature has been
turned on. For more information on the elevated
idle/exhaust restrictor feature, see Starting Your Diesel
Engine on page 2-2.
REDUCED ENGINE POWER
On diesel engines, a computer monitors the operation of
the electric accelerator. If the REDUCED ENGINE
POWER message comes on while you are driving, the
computer is indicating that your vehicle has a problem.
You should take your vehicle in for service soon.
3-8
WATER IN FUEL
This message will come on to warn you if there is water
in the diesel fuel system. For more information on
how this message works, see Diesel Fuel Requirements
and Fuel System on page 5-2.