If your vehicle is showing burning smell like burnt toast or plastic from under the car, here are the most likely causes and the parts you'll need to fix each one.
Driving over a plastic grocery bag or road debris can cause it to stick and melt onto the hot exhaust pipe or catalytic converter, creating a strong, distinct burning plastic smell.
A worn-out clutch slipping under load generates extreme friction and heat. The burning friction material produces an acrid smell that is almost universally compared to burnt toast.
Oil dripping from a failing gasket onto the hot exhaust manifold burns off instantly. While technically burning oil, many drivers describe the harsh, acrid smoke as smelling like burning plastic.
A stuck brake caliper keeps the brake pads pressed against the rotor while driving. The resulting extreme friction smells strongly like burnt toast or burning carpet.
An electrical short circuit or a wiring harness that was improperly routed and is touching the hot exhaust will melt the wire's plastic insulation, causing a severe burning plastic odor.