OK ... back at the workshop now. Yes, test 14 is indeed the switch test (but that may have been a red herring, I reckon).
Meanwhile, test 23 will give you the error codes stack (scroll through with the single down arrow key).
And ... test 24 is the set sensor test. You will need a giving set (or cotton bud, thin pencil etc.
- technically, a "reflecting surface") for this.
According to the manual, persistent "SEt Out" (or P6) errors may be due to:-
1) Set sensor wiring harness open or shorted (check for broken, loose, or disconnected cable assembly).
2) Weak or open set sensor assembly (replace set sensor in front housing).
3) LED set drive signal missing (replace Q2 on logic board).
So, it's all pointing to the set sensor (but check the wiring and connections - J6). This is an infrared detector mounted in the front housing (looking a bit like a screw head, near the top of the giving set path). Its output is monitored at the CPU via input channel 0 (pin 26) of the A/D converter (U11 on the logic board). If the voltage level is below 1.7 volts this is declared as the set not being loaded. A voltage level of 1.7 volts or above is seen as the set being loaded.
There are four wires going (from J6) to that sensor. Blue, Green, Yellow and Orange. I'm not sure which is which, but +5 volts is fed in on pin 4. And pin 3 should be system ground. The LED is across 3 and 2 (cathode at 3, of course). It is driven from Q2 on the logic board. The detector transistor is across 1 and 4. But all that is encapsulated, of course. Have fun!
By the way, I've found a 591 here. Like greeting and old friend, in all its (his) two-tone blue finery!