i have a 2008 silverado and the check engine light came on and is flashing and running like it is only running on 5 cylinders any input?!

 

Count the number of flashes. It should flash a certain number of times (maybe 5) then have a break or pause then flash again (maybe 4) then a long pause and start over again giving you a 54 code. Google the code numbers. Example 54 would indicate fuel delivery problem. If the light is flashing security while it is running it has a problem with the security system. Sometimes you can disconnect the positive side of the battery first let it sit for 20 minutes then reconnect it and it will reset. If that does not do it google how to do a relearn on the PCM and do that. When I do a relearn I take the headlight fuse out so it does not run the battery down and I do not have to listen to the dinging for 30 minutes. If it is not one of these two things you really need some test equipment to dig any deeper. Hope this helps.