FreeRTOS is a real-time operating system with configurable scheduling policies. Its portability and configurability make FreeRTOS one of the most popular real-time operating systems for embedded devices. We formally analyze the FreeR-TOS scheduler on ARM Cortex-M4 processor in this work. Specifically, we build a formal model for the FreeRTOS ARM Cortex-M4 port and apply model checking to find errors in our models for FreeRTOS example applications. Intriguingly, several errors are found in our application models under different scheduling policies. In order to confirm our findings, we modify application programs distributed by FreeRTOS and reproduce assertion failures on the STM32F429I-DISC1 board.