If your vehicle is showing exhaust fumes smell inside the passenger cabin, here are the most likely causes and the parts you'll need to fix each one.
The exhaust manifold or its gasket can crack or warp over time due to extreme heat cycles. This allows raw exhaust gases to escape into the engine bay, which are then pulled into the cabin through the HVAC fresh air intake.
The flexible section of the exhaust pipe or the pipe flanges often rust out or break from engine movement. This vents exhaust under the front of the car where it easily seeps into the cabin.
A leaking valve cover drips engine oil directly onto the hot exhaust manifold. The burning oil creates a strong, acrid, smoky smell that is frequently mistaken by drivers for raw exhaust fumes.
If exhaust work was recently performed, reusing old crush gaskets, failing to properly torque flange bolts, or loosely fitting clamps will cause immediate exhaust leaks under the cabin.
The EGR system routes hot exhaust gas back into the engine to reduce emissions. A cracked EGR tube or blown gasket will vent exhaust directly into the engine bay.
Aerodynamics create a low-pressure zone behind a moving car. If the rear trunk or hatch seal is torn, exhaust from the tailpipe is sucked directly into the cabin (often called the 'station wagon effect').
On turbocharged vehicles, a leak at the turbocharger-to-manifold gasket or downpipe flange will release highly pressurized exhaust gases right next to the cabin firewall.