I have a 01 2500HD 4x4 with the 6.0L engine and my service engine soon light came on.  So I stopped by Auto Zone and they did a test which showed my knock sensor was bad.  I bought the part and was going to do it myself before I foud out I have to remove the intake manifold to replace both of them.  My question is it a life or death situation if I don't change them or can I just get use to the service engine  light on the dash.

Thanks  

This is a good question. That engine uses generation 3 of knock sensing. It also has a fully electronically controlled timing. In theory if the system only retards spark when sesor hears something you could use better gas and get by but if the system uses a default retard of spark curve when there is a sensor failure to prevent motor fuel knock then over all performance will suffer.  See the whole reason that there is even a knock sensor is so you can use 87 octane fuel and happily motor on ignorant of engines true octane needs and also not go to dealer to complain about it knocking either. In a nutshell if it is not knocking then you are kinda okay for now but you are loosing power and MPG because that means it has retarded curve. GM placed sensor in valve lifter galley in generation three designs so that it could hear knock sooner as it was closer to heads and quell knock before you hear it. Up until engines based on old 350 were discontinued after 2000 the sensor was externally located and easy to change. My personal advise here is to bite the bullet and change it. I would also suggest that you change CMP (cam position sensor) while you are working on it since you need to kinda disturb it when you go in to get knock sensor.  

I would also suggest that you change it. This is one of those parts where there really isn't a good reason not to change it (besides cost) and it could cost you more in the long run not to do it.