D27 is the spare unused ADC input. D31 is the ADC input for the A/C pressure sensor.

According to EEHack's definition file for $EE, the A/C Pressure input is a Byte (8 bits) at Offset 26 in a Mode 00 message (F4 00). Pin D27 input would be read as a Byte (8 bits) at Offset 54 in a Mode 00 message (F4 00).