GMC SIERRA 1998 Manual page 154

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This system is called OBD I1 (On-Board
Diagnostics-Second Generation) and is intended to
assure that emissions are at acceptable levels for the life
of the vehicle, helping to produce a cleaner
environment. The SERVICE ENGINE SOON light
comes on to indicate that there is a problem and service
is required. Malfunctions often will be indicated by the
system before any problem is apparent. This may
prevent more serious damage to your vehicle. This
system is also designed to assist your service technician
in correctly diagnosing any malfunction.
~ O T I C E :
If you keep driving your vehicle with this light
on, after a while, your emission controls may not
work as well, your fuel economy may not be as
good and your engine may not run as smoothly.
This could lead to costly repairs that may not be
covered by your warranty.
This light should come on, as a check to show you it is
working, when the ignition is on and the engine is not
running. If the light doesn't come on, have it repaired.
2-70
This light will also come on during a malfunction in one
of two ways:
0
Light Flashing
-- A misfire condition has been
detected. A misfire increases vehicle emissions and
may damage the emission control system on your
vehicle. Dealer or qualified service center diagnosis
and service may be required.
0
Light On Steady
--
An emission control system
malfunction has been detected on your vehicle.
Dealer or qualified service center diagnosis and
service may be required.
If the Light Is Flashing
The following may prevent'more serious aamage to
your vehicle:
Reducing vehicle speed.
Avoiding hard accelerations.
0
Avoiding steep uphill grades.
0
If you are towing a trailer, reduce the amount of
cargo being hauled as soon as
it
is possible.
If the light stops flashing and remains on steady, see "If
the Light Is On Steady" following.
If the light continues to flash, when it is safe to do so,
stop the vehicle. Find a safe place to park your vehicle.

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