code 81 is lift pot out of range

these potentiometers look for a voltage range when operating, the truck manager looks at the voltage to make sure it is centered in that range (in neutral position).
The range these usually look for is between 0.156v and 4.84v for the lift and throttle pots.

Generally when installing a potentiometer in one of these you would need to have the truck already in maintenance test mode for monitoring the voltage of the respective potentiometer. Then unplugging the handle and performing the pot change out, then plugging it back in and setting the pot at the correct voltage range.
THEN running a relearn test on the handle to calibrate the pots to the TMM.
You can also try and use a VOM and guess at the 'center' range using the ohm meter but you would still need to have the units TMM in test mode for the respective pot to make sure it is centered and adjust as needed.

the problem most tech's have when installing these pots is once you get this set and start up the truck, if it does not see the potentiometer centered it will error out as you can see, and getting the truck to enter into any mode for testing or learning will be a challenge as you will have to try and GUESS at where the center is by holding the handle in that spot to prevent it from erroring out while trying to navigate/activate the menu system. a very challenging thing to do. Once you successfully run the learn process on it then the error will go away ;)