What could cause this?

Looking at a 2019 Ford Focus ST 2.31 ecoboost manual with 28k miles. Checked it's mot history to see it failed in May 2024 due to -

Exhaust carbon monoxide content after 2nd fast idle exceeds default limits (8.2.1.2 (b

Basically wondering how serious this could be and what I need to consider. It passed it's mot in 2025 and would come with a new mot