GMC Troubleshooting Questions? Ask a Mechanic for Answers
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What I have seen cause this here is the switch itself fails in some cases you can remove the interior tail gate paneling to access the connection to the switch and it may be corroded or have fallen off to where you know for sure it needs replaced.The other issues are the 3 latches on this model that lower and swing the tail gate.GM had released a bulletin on the 3 latches for this door failing and to replace them with the updated latches to correct faults like this as well.
The way to check to see if the latches is the cause or the switch (if no visible corrosion or damage is found) is to use a Tech 2 scan tool or similar that can actuate the latches to see if the tail gate swings as well as lowers.If it does then the latches are o.k. and the switch needs replaced.If the latches do not release or one or more hang up then the latches need replaced with the updated latch assemblies to correct this here.
If you cannot get the inner paneling off of this door due to it being stuck closed a scan tool like the Tech 2 scan tool plugged into your vehicle will be needed to actuate the latches so you can open it to remove the interior paneling.
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