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SUMMARY This paper presents a novel and cheap methodology to classify healthy and clogged air filter. Air filter is an integral part of air intake system of spark ignition engine and is responsible to deliver clean air for combustion process. A clogged air filter may hamper engine power and its drivability performance. As a consequence, its health monitoring becomes mandatory. This task is accomplished by modeling the air filter effects on air flow through inlet manifold pressure by incorporating a newly introduced air filter discharge coefficient ( C af) in its dynamics. The estimation of C af gives an idea about the health of air filter, as no sensor can be installed to measure it. A second-order sliding mode observer is employed to estimate immeasurable C af. Super twisting-based sliding mode observer requires manifold pressure, engine angular speed, and load torque as input. A successful implementation has been carried out to diagnose clogged and healthy air filter of commercial vehicle engine compliant to on-board diagnostics version-II. This characterizes ' C af' as a clean indicator of air filter health classification. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]