2008 GMC Envoy . . Trouble Codes P0306 and P0171
P0306 is a faulty ignition coil #6 or a bad spark plug. Switch the #6 coil with #4 coil and see if you get a new code P0304 and you know the coil is bad vs the plug. P0171 is a lean condition, maybe related to P0306.
Check for a vacuum leak. If you have over 100,000 miles on your Envoy you're due for a tune up. HTH. -Jim
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