Which factors could result in a rich running engine?

A rich running engine occurs when there is an excess of fuel in the air-fuel mixture that is delivered to the engine's combustion chamber, leading to inefficient combustion and potentially causing performance issues and increased emissions. Each of the factors mentioned contributes to this condition in different ways.

A faulty oxygen sensor can lead to an incorrect reading of the oxygen levels in the exhaust. If the sensor misinterprets the amount of oxygen present, the engine control unit (ECU) may compensate by injecting more fuel into the intake manifold, resulting in a rich mixture.

A dirty air filter restricts airflow to the engine. When the engine receives insufficient air, the fuel mixture becomes proportionately too rich because the same amount of fuel is being delivered without the necessary amount of air for optimal combustion.

Clogged fuel injectors can cause a problem as well. If the injectors are obstructed or malfunctioning, they may deliver an excessive amount of fuel, either by not atomizing it correctly or by allowing too much fuel to flow through, leading to a rich mixture.

Considering these factors collectively illustrates how each one plays a role in creating an environment for a rich-running engine. Therefore, the correct answer encompasses all of these conditions occurring individually or in combination.