Katha-1-3-7-yastvavijñānavā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Now the text mentions the results produced by the driver (intelligence) who is unknowing, as above stated; the lord of the chariot, who does not know and who has not the mind under control and who is, therefore, always unclean, does not attain that immortal great goal already described, by reason of having such a driver. It is not alone that he does not attain that, but he reaches Samsâra marked by births and deaths.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda