Severity will moderately high here. Reading this, a certain sense of urgency needs to be applied in order to maintain efficient engine operation. Anytime a fault can directly affect very important values such as air/fuel ratios, and is actively present, you should not operate your vehicle, to prevent any engine damage. Having that said, if you've driven the vehicle since the fault has been active, don't worry too much, you are probably fine. Big takeaway, if left unattended, this fault could cause expensive internal engine damage in the future.