I have a 2007 siverado 5.3L . I had a block heater installed but when I plug it in at neg10 degres when started the dash reads engine over heated. The dealer said it was to warm to plug in. The factary cord has a temp sensor in it so it will only come on at 0deg or less?[][]

 

GM has been using block heater cords with a thermostatic cutoff switch in the plug end of the cord.
If this thermostat doesn't cutoff power .... the block heater warms the engine too much.
When this happens you could end up with error codes indicating overheating and starting issues.

If the cord end is in the cold wind the thermostatic cutoff may not warm enough to work. Relocating it to an engine bracket or coolant hose will help this thermostat in the cord stay warm and help with better operation.