: To clear codes, navigate until "CLr?" appears and use the hazard flasher switch or "Set/Save" key to confirm. On skid steers, you may need to hold the "SELECT" button for 5 seconds when "WAIT" is displayed. How to retrieve codes on a John Deere 6420, 6000 series

With the key OFF, disconnect the main ECU and the suspected secondary ECU. Using your multimeter on the ohms setting, probe the CAN HI and CAN LO wires (typically twisted yellow and green wires). You should read approximately (two 120-ohm resistors in parallel). An open loop (OL) means a broken wire or failed resistor.

One final nuance: Some John Deere displays (GreenStar, CommandCenter Gen 4) have reported D1A as a with no active fault and no performance impact. This is typically a display firmware bug where a momentary voltage dip (e.g., cranking the starter) triggers a one-time D1A that never reactivates.

In plain English: The engine control unit (ECU) is receiving a signal from the DPF differential pressure sensor, but that signal doesn’t make physical sense given what the engine is doing. It’s not that the sensor is definitely broken; it’s that the story the sensor is telling contradicts other known data (like exhaust flow, RPM, or load).

: When "dIA" appears, it signifies the machine is ready to display stored Diagnostic Trouble Codes (DTCs) from various control units.

: Use the arrow switches or turn signal lever to scroll through control units (e.g., ECU, BCU).

When this code appears, the machine may disable certain functions, such as the transmission or reverser lever, to prevent operation while in a diagnostic state. It typically surfaces for one of three reasons: