# How to fall back PX4 bootloader from px4\_fmu-v3 to px4\_fmu-v2?

**URL:** <https://discuss.px4.io/t/how-to-fall-back-px4-bootloader-from-px4-fmu-v3-to-px4-fmu-v2/13116>\
**Category:** Pixhawk\
**Created:** [September 27, 2019, 12:50pm UTC](https://discuss.px4.io/t/how-to-fall-back-px4-bootloader-from-px4-fmu-v3-to-px4-fmu-v2/13116 "2019-09-27T12:50:31Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![shelley](https://discuss.px4.io/user_avatar/discuss.px4.io/shelley/32/5700_2.png) [@shelley](https://discuss.px4.io/u/shelley)\
**Post date:** [September 27, 2019, 12:50pm UTC](https://discuss.px4.io/t/how-to-fall-back-px4-bootloader-from-px4-fmu-v3-to-px4-fmu-v2/13116/1 "2019-09-27T12:50:31Z")

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Hi all,  
I connected pixhawk2.4.8( with the Firmware of PX4 1.9.1, px4\_fmu-v3\_default) to QGC through USB port on my computer.  
But QGC says “Error on link Serial Device on ttyACM0. Error connecting: Could not create port. Permission error while locking the device”.  
Do anyone can fix this issue?  
Or just fallback to px4\_fmu-v2\_default?  
How to download a fmu-v2 bootloader to a pixhawk2.4.8( with the Firmware of PX4 1.9.1, px4\_fmu-v3\_default)?

Thanks&Best Regards!