Hope that you have your problem sorted by now; I haven't been around for some time.

The EGR (Exhaust Gas Recirculation) Valve is the valve which bleeds exhaust gasses from the exhaust manifold to the air intake in order to dilute fuel/ air ratio and reduce ignition temperature and thereby reducing NOx emissions. When this valve fails, the engine stops reacting correctly to ECM instructions and a side effect confuses the Transmission Control Module.

If you had the other symptoms I described, this may well be your problem, but it would seem that PRND lighting simultaneously is a general warning for transmission problems.

Hello, I saw your post about the PRND lights being lit up all the time and that you cured it by replacing the EGR valve. I was hoping for some clarification, could you please email me at legendrx7@hotmail.com or respond here. My question at this point is....When you refer to the EGR valve, are you talking about the valve that the airpump pumps air into?

My 04 CTT is doing the exact same thing with the PRND lights.