If you have not looked at this site ....you need to!!!! read the whole thing!!
How to - Upgrade to a 996 Instrument Cluster - How To - www.BoXa.net
error is most like due to the fact you do not have the 2 wire sender unit, 1 for the light and the other for the gauge. He suggested a fix for that short of putting in the correct sender and running the extra wire. But first see if his first suggestion works. Jeff
You need to make the cluster think it has an oil pressure sender. When the car runs there is a resistence of 200-1500 ohms. Buy one of the yellow VW wires like you used for your trip computer retrofit. Cut it in half and solder inline a 500 ohm resistor. One end of the wire goes in the connector to the cluster - just like when you did the trip computer. It goes in the blue connector - slot 5 - which should be unused as you do not have a pressure sender. Ground the other end of the wire. Peter did not mention any special place for the ground, so try the same place you used for the ground wire when you did the trip computer retrofit. No one has done this so I hope it works
.B5 on you connector should be empty as that is for the oil pressure sender wire that the Boxster does not have. That is where you will try Peter's hack.
B2 on your connector should have a wire since it is the same for a Boxster/996/turbo.
DME CODING. A procedure has been used to successfully code a 986 DME as a 996, code the instrument cluster to C2 Cabriolet and finally recode the DME back to 986. I would strongly suggest that the person following this procedure be an experienced PST2 / PIWIS operator. There are several steps (key strokes) needed to accomplish the coding.
http://www.boxa.net/forum/index.php?showtopic=62435
This is the most complete write up I have seen yet.
They talk about gauges BUT also cruiser control.
They say cruise has to be programed in BOTH the DME and the cluster.
This seems to match what the Porsche dealer said to me today when they returned my car.
I'll try and give you some more up-dates and pictures over the weekend.