"...The piston can overheat and knocking can and will occur. This knocking is caused by ignition of the gas/air mix in your cylinder before the spark of the spark plug. This causes temperature and pressure peaks that will melt your piston, crack your cylinder head, and drop one of your exhaust valves. You have heard about those abrupt deaths of air cooled engines..."
*EDIT: THE KNOCK SENSOR DOES NOT DO THIS, but what I have said about this Chemistry stuff is true. The EGR valve lowers the temperatures inside the combustion chainber so that the NOx emissions are reduced.
See the NOx sensor picks up knocking because the cylinder walls and piston get really hot because of the lean fuel mixture which makes the bonds of N2 which is regularly found in air at close to 80% of air to combine with O2 to for N20, NO, N2O5, and other NOx mixtures. Instead of N2 being a non reacting molecule passing through the engine, it reacts with air at high temperatures to form N20, NO, N2O5, etc. Basically N2 + O2 ~~~> NOx emissions, which is an endothermic reaction (requires heat to go).
SL600 1993 has 11:1 pistons, geez...make sure you run 92, 94, 98 octane gas...maybe you got a slug of bad gas.
But lets get to the real problem. The SL600 isn't running properly, if it knocks, like pinging noises, its running too lean.