What causes P1811 code?
A P1811 diagnostic trouble code means the hydraulic line pressure has been raised to compensate for excessively worn clutches or other components. If left unchecked, this excessive line pressure can cause serious damage to other control components, making a small problem a lot worse.
What is the difference between a 4T65E and an 4T65E HD?
Physically they are about the same and internally identical but there is really not much at all that interchanges between the two. The big enhancement to the 4T65E was that it uses a pressure control solenoid so the pcm can control line pressure instead of using a vacuum line like the older 4T60E.