Chevrolet Forum - Chevy Enthusiasts Forums - "Reflash" of ECM by Dealership

I have a 2004 Silverado LS 5.3L (2wd) that I purchased used back in 2010. For the most part, it has been a decent truck. Recently, my "Check Engine" light came on and went off every couple days -- finally staying "on" all the time about two weeks ago. The truck runs fine and my local mechanic ran a diagnostic on it (at NO charge to me - unbelievable) last week. It showed all four oxygen sensors as being faulty, which he knew was not true. He said I need to take it to a Chevy/GM dealer and have the ECM "reflashed" before the light will go off......and THEN can take action to reset it or replace any faulty sensor.

I have 136,xxx miles on the truck and it runs strong. I am on a 'staycation' from work this week, so I have time to wait on it in their lobby if need be.

How much do you think a dealer will charge for such a task? I can't imagine it taking more than one hour of labor to do it. Thanks for any information or advice!!