As per the title I am not a mechanic so if any one wants to add anything or correct me please feel free to post up.
I recently had the sdl problem which seems to be a part of the ownership experience with any car that has one fitted.
First you need to see if your problem is actual cat/catbypass overheating or an indication fault, to do this you need to check the cat / catbypass pipe temperatures with a thermometer, obviously if the cats are too hot this is the problem.
My car is a 5.2 355, this has one thermocouple and ecu per side with one sdl light, I believe the 2.7 355 has has one thermocouple and ecu per side with two sdl lights. the U.S.A version has a third thermocouple and ecu in the 'y' pipe I believe.
The ecu's can be found by following the steel braided cables from the thermocouples on the cats to the ecu's. one ecu is by the lhs suspension top mount and the rhs is under the air pump behind the rear bumper.
Disconnect the two pin thermocouple plug from the ecu and test it with a multimeter on ohms or resistance scale. Both my thermocouples read the same on a multimeter i.e. about 2 ohms with a cold exhaust and about 22 ohms with a hot exhaust on tickover. Apparently the thermocouples generate a millivolt signal (I dont know the correct values) but low ohmic values similar to the above should indicate healthy thermocouples.
The 3 pin ecu plug should have the following d.c. voltages, 12v between the two power wires, and the following voltages to earth on the third signal wire;
0.5V @ 300C
4.5V @1100C
5V with thermocouple disconnected (thanks to eric 355 for these values)
My car was about 0.5v d.c. cold and about 1.0v d.c on tickover. Next I connected a short wire from the signal wire with a 'scotchlock' and cable tied it to something to make testing easier.
As my problem was intermittent I went for a drive until the sdl light came on permanently and shut down one cylinder bank. At this point I stopped and checked both signal wire outputs with the engine still running, the r.h ecu was outputting about 5.0 volts, over the 4.5v max. I then unplugged the thermocouples and checked them again, they were close to the above values so the r.h.s. ecu was the culprit with an intermittent fault.
A new ecu has been ordered from Eurospares, I believe the later ones are designed better.
Hope this helps other owners with this problem, please feel free to p.m. if you need any more info.