If your vehicle is showing exhaust smells strongly of unburned fuel or rotten eggs, here are the most likely causes and the parts you'll need to fix each one.
A failing O2 sensor can falsely report a lean condition, causing the engine computer to dump excess fuel into the cylinders. This unburned fuel creates a strong gas smell and overworks the catalytic converter, leading to a rotten egg odor.
The catalytic converter is responsible for neutralizing exhaust gases. When it fails or becomes clogged, sulfur in the fuel isn't processed correctly, resulting in a distinct rotten egg smell.
The MAF sensor measures air entering the engine. If dirty or failing, it can miscalculate the air volume, leading to a rich fuel mixture and unburned fuel in the exhaust.
If a spark plug or ignition coil fails to ignite the air/fuel mixture, raw fuel is pumped directly into the exhaust system. This causes a heavy fuel odor and can rapidly destroy the catalytic converter.
A fuel injector that is stuck open or leaking will spray continuous fuel into the cylinder, creating an extremely rich condition that the engine cannot burn off.
If this sensor fails and reports that the engine is always freezing cold, the computer will continuously run a rich 'warm-up' fuel mixture, leading to unburned fuel smells.
A leak in the exhaust manifold before the O2 sensor can draw in outside air. The sensor reads this as a lean condition and commands extra fuel, causing a rich mixture.