May be manufacturer-specific, but for GM, Ford and Isuzu at least the OBD code P1860 is "TCC PWM solenoid electrical circuit" -- the torque converter clutch application solenoid. It controls when and how firmly the clutch applies. The Ford entry is a little longer, adding "transmission transfer case contact plate 'D' short circuit to battery."
I also found the same code on Audi and Volkswagen models described as "kick down switch error message" so if this is on a German car the following may not help...
Have you noticed the car jerking at speed -- say maybe 40-50 mph? That would likely be the fluid clutch locking and unlocking. Or maybe you've noticed engine rpm higher than normal at a given speed, or fuel mileage decreasing -- that would indicate it isn't locking.
Problem could be the solenoid, or the wiring to it. From what I have read, the code gets set if there is either a continuous open circuit or a short in the circuit. The solenoid mounting will depend on manufacturer/model/year, but typically seems to be either under an inspection plate or else accessed by removing the transmission oil pan.
