GMC Troubleshooting Questions? Ask a Mechanic for Answers
as you have a reduced engine power message then there should be an error code you can pull off the ECU with a scan tool
this is by far the fastest way to get to the bottom of it
it’s also worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.
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