Chevrolet Repair: chevy tahoe, chevy tahoe, diagnostic checks

Question
hello, i have a chevy tahoe 2003 and the reduced engine power light came on, then the car almost shut down, then after turning the car off it is not getting gas or something. everything works except that i called onstar and after they did a diagnostic they stated the car needs to immediately be towed to a service station. what could be wrong and is it going to be very expensive?

Answer
There is no way I can answer this as I am not familiar with onstars diagnostic abilities. I would have to research their capabilities. I have never heard of Onstar running diagnostic checks before.
For them to say immediately might just be a term that they are required to say.