Build the home brew tester at the very bottom of this link, pull the codes and use that site to determine what the codes mean:

K6JRF Auto Page

The Electronic Throttle Actuator (ETA) controls the Idle Sped Control, but if it's the original harness, I think most people here would agree that it is most likely in need of replacement. I would start with the harness after the cheaper stuff like the fuel filter, fuel pressure regulator, stop lamp switch and neutral safety switch.

Incidentally, I had a 1993 400E w/out ASR and I remember that for some strange reason my idle got lower and lower over a period of several years. So low that it occasionally stalled while moving from reverse to drive; but after changing the motor mounts the problem totally corrected itself. How the two were related is beyond me. Maybe it was just a coincidence.

I think all the 400E's were LH (hot wire).

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles