If your vehicle is showing seatbelt warning light stays on when buckled, here are the most likely causes and the parts you'll need to fix each one.
Dirt, coins, or food crumbs can get lodged inside the buckle receptacle. This prevents the metal seatbelt tongue from fully depressing the internal microswitch, even though it clicks into place.
The internal electrical switch inside the seatbelt buckle wears out over time from repeated use. When it fails, it cannot break the electrical circuit to tell the car's computer that the belt is buckled.
Modern vehicles have an occupant classification system in the passenger seat. Placing a heavy bag, groceries, or a pet on the seat will trigger the sensor, turning on the warning light because that seat's belt is unbuckled.
The wiring harness under the seat that connects the buckle to the vehicle's computer can be kicked loose by rear passengers, snagged by debris, or left unplugged after a recent interior detail or seat removal.
The main computer responsible for processing interior electronic signals, including the seatbelt circuit, has failed or experienced a software glitch. This is a rare electrical failure.