What indicates that a diesel engine air filter needs changing?

A diesel engine air filter needs changing when the restriction gauge indicates it is beginning to plug because this gauge provides a direct measurement of airflow resistance through the filter. As the filter accumulates dirt and debris, airflow becomes restricted, which can negatively impact engine performance by reducing the amount of clean air entering the combustion chamber. When the gauge shows that the filter is reaching a critical level of restriction, it is a clear and quantifiable signal that maintenance is required to avoid potential engine issues caused by insufficient air supply.

While a dirty appearance of the filter and age of the filter can be indicators, they are not as reliable as the restriction gauge data. A filter may appear dirty yet still function adequately, and not all filters need to be replaced strictly based on age, as usage and environmental conditions play a significant role in determining their lifespan. Engine noise may also be a sign of other issues but does not specifically indicate the need for a filter change.