The most common culprit. The transponder in the key requires power to send its authentication signal. If the battery voltage drops below ~2.5V, the signal becomes too weak. The ECU interprets this as a "missing key."

Often, the D1A code appears simply because the DEF tank was run low, and the sensor lost its reference.

: Press Enter or Select on a specific unit to see the active or stored codes.

Cycle the key and then ON again to see if the code clears. 3. Navigate the Diagnostic Menu