Lamp energy voltage is measured during the cell blank check:

U   Lamp energy too high

Obtaining lamp energy from a cuvette at each of the 14 wavelengths  greater than 9.1000 volts, prints the "U" flag and prints the cell number after the cell blank measurement.

D   Lamp energy too low

Obtaining lamp energy from a cuvette at each of the 14 wavelengths less than 4.0000 volts, prints the "D" flag and prints the cell number after the cell blank measurement.