Found online:
I have a 2001 5.3 silverado, i found on another board that this could mean that the knock sensor is faulty, probably from getting wet, i guess they have to be replaced and its kinda a pain from what i read, you have to take off the intake manifold to access them,
Another post with more detailed info:
GM developed a procedure for replacing the knock sensors a while ago.
It involves replacing both knock sensors and both wiring harnesses going to the sensors.
You then build a small mountain of RVT under the intake manifold to divert the water away from the sensors wire boots, you might even need to remove the foam from the one end of the intake manifold.
You then replace both intake manifold gaskets and torque the intake manifold to INCH POUNDS and not foot pounds.
If you mark all the wiring before you remove it and put it back together in the same order as what you took it apart, you should not have any problems.
If the check engine light was not on before you started to do the power tune, then chances are, unless the ports where the knock sensors were, was full of water, and unless the terminals were corroded, the problem was not with the knock sensors to begin with - but with the person / equipment that was doing the power tune.
