The John Deere Diagnostic Trouble Code (DTC) D1A refers to an issue with the Auto Idler Solenoid Circuit.
Step 5: Software Update In rare cases, this code appears due to a software glitch. Check with your dealer to ensure your Gen 4 display has the latest firmware (version 19.2 or newer is generally safe).
Procedure for dealing with diagnostic trouble codes - OMLG100524 John Deere How to retrieve codes on a John Deere 6420, 6000 series
You will usually see this alongside companion codes, such as D1A01, D1A02, D1A03, or D1A04.