Boxster fault codes
 

My 1997 Boxster keeps getting codes P0140, P0154 and P1117. Oxygen sensors have been replaced 3 times. Engine runs fine with no symptoms of anything being wrong. Since this always concerns the same bank, can I expect wiring is the problem? If not wiring what would cause CEL to come on? Total mileage has been less than 3000 miles over 5 years.

The reason that replacing the sensors did nothing is because the P0140 and 0154 are for interruption of signal from the sensors, which is often not the sensor but a harness issue. P1117 is for a low voltage to sensor ahead of the three way cat on bank one, which is also typically a harness issue. Before replacing ANY sensors, particularly expensive ones, find out what the codes actually mean.

You might consider checking for mouse droppings in the engine bay, or around the car. If none show up, wonder if the car still has the original ignition switch, which is a blossom of electrical gremlins.
Regards