Experiencing a bad smell when sneezing for two years is usually caused by a chronic sinus infection (sinusitis), post-nasal drip, nasal polyps, or sometimes an underlying dental issue. Please don't worry, as these conditions are very common and easily manageable. I highly recommend visiting an Ear, Nose, and Throat (ENT) specialist for an accurate diagnosis. For temporary relief, you can use generic saline nasal rinses to flush out the nasal passages. Depending on what the specialist finds, treatment may include nasal corticosteroid sprays (such as fluticasone) or antibiotics if a bacterial infection is present.