How many flashes?
From the manual...
Your locomotive includes built-in diagnostics to monitor the condition of the main drive motor and smoke system. 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.
Note that smoke-related errors can be reset a maximum of three times. On the third try, if the problem still persists, the smoke unit will be shut down and must be returned to Lionel Service for repair.
Turning the smoke unit switch to the off position will disable diagnostic checks of the smoke system.
Number of Blinks Diagnostic Code Description
1 Main drive motor stalled.
2 Smoke 1 element problem.
3 Smoke 1 fan problem.
4 Not applicable.
5 Not applicable.