In the most frequent interpretation, on ECU 1. In some models, it may specify "Analog Sensor Supply Voltage Fault." However, the majority of technical service bulletins (TSBs) from John Deere confirm that D1A activates when the ECU detects that the primary battery voltage has dropped below 10.5 volts for a defined period while the key is on.
: On certain model variations, physical rocker or toggle switches under the console menu trigger this data mode. If a switch shorts out or sticks down, the cluster displays the mode automatically.