Page 43 of the manual covers this https://www.lionelsupport.com/...d-NonPwd%20Units.pdf
Locomotive diagnostics
our locomotive includes built-in diagnostics to monitor the condition of the main drive motor. If
a problem is detected, the locomotive's cab light will blink a diagnostic code.
If you see the light inside the cab flashing, press RESET (for LEGACY operation), 0 (for
TMCC operation), or DIRECTION/power interruption (for conventional operation to attempt to clear the problem. The locomotive will immediately check itself again. If the problem persists, the cab light will blink the code again.
Number of Blinks Diagnostic Code Description
1 Main drive motor stalled.