GMC Sierra 2008 Owner's Manual page 385

4. If the CHECK TRAILER WIRING message
re-appears, the electrical fault is on the
vehicle side.
If the CHECK TRAILER WIRING message only
re-appears when you connect the trailer wiring
harness to the vehicle, the electrical fault is on
the trailer side.
SERVICE TRAILER BRAKE SYSTEM – This message
will be displayed when there is a problem with the
ITBC system. If this message persists over multiple
ignition cycles there is problem with the ITBC system.
Take your vehicle to an authorized GM dealer to
have the ITBC system diagnosed and repaired.
If either the CHECK TRAILER WIRING or SERVICE
TRAILER BRAKE SYSTEM message is displayed
while you are driving your vehicle, you no longer
have power available to your trailer brakes.
When traffic conditions allow, carefully pull your vehicle
over to the side of the road and turn the ignition off.
Check the wiring connection to the trailer and turn the
ignition back on. If either of these messages continues,
either your vehicle or your trailer needs service.
An authorized GM dealer may be able to diagnose
and repair problems with your trailer. However,
any diagnosis and repair of your trailer is not covered
under your GM Warranty. Please contact your trailer
dealer for assistance with trailer repairs and trailer
warranty information.
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