For 1990-2009 cars

Cause | Correction
--- | ---
Stuck valves (Carbon buildup on the valve stem can cause the valve not to close properly.) | Repair or replace as required.
Excessively worn or mis-aligned timing belt | Replace the timing belt and sprockets as required. Refer to Timing Belt Replacement.
Worn camshaft lobes | Replace the camshaft. Refer to Camshaft Replacement.
Improper valve lash adjustment | Refer to Valve Clearance Adjustment.