What is the most common failure that cause sporadic appearance of fault "rich mixture'' ?

Old or dirty air filter ( need replacement) / blocked fuel filter as well.

faulty O2 sensor too.

dirty air filter- leaky fuel injector-A bad spark plugs or ignition coils-Stuck open injectors -bad fuel

faulty oxygen sensor.

fault air intake pressure sensor

Incorrect timing adjustment

bad injectors

leak in the air circuit.

dirty air filter – Stuck open injectors – Bad oxygen sensors – Faulty ECU – Bad Mass Airflow sensor

1- A dirty or leaky fuel injector . 

2- A bad spark plugs or ignition coils . 

3- A so dirty air filter "clogged" .

Common failure cause of rich mixture is that a plugged Air -Filter,  fuel pressure regulator leaking,and leaking injector.