diff --git a/ROMFS/px4fmu_common/init.d/rc.sensors b/ROMFS/px4fmu_common/init.d/rc.sensors
index 7c165476fd..eee59a2ea5 100644
--- a/ROMFS/px4fmu_common/init.d/rc.sensors
+++ b/ROMFS/px4fmu_common/init.d/rc.sensors
@@ -183,6 +183,10 @@ then
 	then
 	fi
 
+	# Start either MPU9250 or BMI160. They are both connected to the same SPI bus and use the same
+	# chip select pin. There are different boards with either one of them and the WHO_AM_I register
+	# will prevent the incorrect driver from a successful initialization.
+
 	# Internal SPI bus mpu9250 is rotated 90 deg yaw
 	if mpu9250 -R 2 start
 	then
diff --git a/src/drivers/boards/px4fmu-v4/board_config.h b/src/drivers/boards/px4fmu-v4/board_config.h
index 8309cb264f..1b3d980910 100644
--- a/src/drivers/boards/px4fmu-v4/board_config.h
+++ b/src/drivers/boards/px4fmu-v4/board_config.h
@@ -84,6 +84,9 @@
 #define GPIO_SPI_CS_LIS3MDL		(GPIO_OUTPUT|GPIO_PUSHPULL|GPIO_SPEED_2MHz|GPIO_OUTPUT_SET|GPIO_PORTE|GPIO_PIN15)
 #define GPIO_SPI_CS_MS5611		(GPIO_OUTPUT|GPIO_PUSHPULL|GPIO_SPEED_2MHz|GPIO_OUTPUT_SET|GPIO_PORTD|GPIO_PIN7)
 #define GPIO_SPI_CS_ICM_20608_G (GPIO_OUTPUT|GPIO_PUSHPULL|GPIO_SPEED_2MHz|GPIO_OUTPUT_SET|GPIO_PORTC|GPIO_PIN15)
+
+/* The BMI160 sensor replaces the MPU9250 on some boards. Only one is actually present and connected
+ * to the second GPIO pin on port C. The wrong driver will fail during start becaus of an incorrect WHO_AM_I register.*/
 #define GPIO_SPI_CS_BMI160    (GPIO_OUTPUT|GPIO_PUSHPULL|GPIO_SPEED_2MHz|GPIO_OUTPUT_SET|GPIO_PORTC|GPIO_PIN2)
 
 #define GPIO_SPI_CS_FRAM		(GPIO_OUTPUT|GPIO_PUSHPULL|GPIO_SPEED_2MHz|GPIO_OUTPUT_SET|GPIO_PORTD|GPIO_PIN10)