GMC Troubleshooting Questions? Ask a Mechanic for Answers
It really depends on what system is triggering the code, but usually the only thing that can be affected by weather like that would be either a temperature sensor or a fuel pressure sensor (fuel expands more when it's warm). In my opinion the most likely cause is a stuck open thermostat which only triggers the code when cold because the engine doesn't think it's heating up to operating temperature quickly enough (caused by a stuck open thermostat).
Is there any way you could get the vehicle scanned for trouble codes?
I like the Lucas stop leak product. It seems to do a good job without gumming anything up.
Please let me know if you need additional info.