The simplest reset method is a full power cycle:

Inspect the area around the diagnostic connector for a small jumper wire. The diagnostic socket is usually located near the fuse box, under the instrument panel, or behind a cover on the right‑hand side of the operator’s station. If you find a jumper plug inserted into the socket, remove it. The D1A code should clear within a few seconds of removing the jumper. If you are unsure what the jumper looks like, look for a small black or white plastic body with two wires exiting it that ends in a pair of terminals designed to fit into two adjacent sockets of the diagnostic connector.

Insert a standard spare fuse into this slot to bridge the connection.

Turn the ignition key to the position without starting the engine.

Usually, an operator intentionally forces the tractor into "d1A" mode by installing a dedicated diagnostic fuse. However, if your tractor suddenly enters "d1A" mode while driving—accompanied by flashing warning icons or an immediate loss of transmission drive—you are dealing with an electrical short. This typically indicates that a wiring harness has rubbed raw, or water has bridged contacts inside the fuse block, fooling the tractor into thinking the diagnostic bridge has been installed. Step-by-Step: How to Manually Access Codes via "d1A"

However, to get the machine back to the barn or shop:

Translation: Your engine oil pressure is dangerously low. Stop the engine immediately. FridayParts

The D1A code is a severe fault code indicating a scenario. In the context of John Deere's diagnostic system, this is not a warning to ignore; it is a critical alert designed to prevent catastrophic engine failure.