Apple diagnostic PPN001. Fan running high, computer slow

Apple Hardware Test – Invoke and Interpret

Apple Hardware test – How to run and interpret (2)

Apple Diagnostics: Reference codes

Restart the computer holding down the command - option/alt - P - R keys before the gray screen appears. Continue to hold until you hear the startup chime for the second time. You may need to do this 2-3 times.

Reset NVRAM/PRAM

Try a SMC reset. The article covers when this is required, but doing this will not affect anything.

Reset SMC