> For the complete documentation index, see [llms.txt](https://knowledge.illumina.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://knowledge.illumina.com/instrumentation/novaseq-x-x-plus/instrumentation-novaseq-x-x-plus-troubleshooting-list/000007997.md).

# How to resolve 'An unknown error has occurred' critical system errors on the NovaSeq X Series Instruments

NovaSeq X Series Control Software initialization issues that do not fit within the Control Software error reporting structure will present with the following error message:

![](/files/GRgpoyUThyW0goG3SFSq)

"Critical system error - An unknown error has occurred. The system may require a restart."

If this error presents during instrument or Control Software initialization, see the following recommendations:

**Troubleshooting Actions**

1. Do not select Shutdown or power cycle the instrument. Instead, select **Exit Application** to close the NovaSeq X Series Control Software.
2. Collect the Control Software Logs from the /usr/local/illumina/logs/novaseq-x-series-control-software directory on the instrument.
3. Contact [Illumina Tech Support](https://www.illumina.com/company/contact-us.html) for next steps.

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| *For any feedback or questions regarding this article (Illumina Knowledge Article #7997), contact Illumina Technical Support* [*techsupport@illumina.com*](mailto:techsupport@illumina.com?subject=Question%2FFeedback%20Regarding%20Illumina%20Knowledge%20Article%20#000007997%20-%20Instrumentation%20\&body=Dear%20Illumina%20Technical%20Support,%0D%0A%0D%0A)*.* |


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