GMC Yukon XL Denali Owner's Manual page 315

If the disconnect occurs while
the vehicle is stationary, this
message will automatically turn
off in about thirty seconds. This
message will also turn off if the
driver acknowledges this
message off or if the trailer
harness is re-connected.
If the disconnect occurs while
the vehicle is moving, this
message will continue until the
ignition is turned off. This
message will also turn off if the
driver acknowledges this
message off or if the trailer
harness is re-connected.
2. There is an electrical fault in the
wiring to the electric trailer
brakes. This message will
continue as long as there is
an electrical fault in the trailer
wiring. This message will
also turn off if the driver
acknowledges this message off.
To determine if the electrical fault is
on the vehicle side or trailer side of
the trailer wiring harness
connection, do the following:
1. Disconnect the trailer wiring
harness from the vehicle.
2. Turn the ignition OFF.
3. Wait ten seconds, then turn the
ignition back to RUN.
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
connecting 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 the vehicle
Driving and Operating
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 driving the vehicle,
power is no longer available to the
trailer brakes. When traffic
conditions allow, carefully pull the
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
the vehicle or trailer needs service.
An authorized GM dealer may be
able to diagnose and repair
problems with the trailer. However,
any diagnosis and repair of the
trailer is not covered under the
vehicle warranty. Please contact
your trailer dealer for assistance
with trailer repairs and trailer
warranty information.
9-71