If your vehicle is showing sudden and severe drop in gas mileage, here are the most likely causes and the parts you'll need to fix each one.
The primary sensor for monitoring fuel mixture. If it fails and falsely reads a 'lean' condition, the engine computer will continuously dump excess fuel into the cylinders to compensate.
If the thermostat is stuck open or the temperature sensor reads cold, the engine never reaches operating temperature. The computer keeps the vehicle in a fuel-heavy 'warmup' mode indefinitely.
This sensor measures air entering the engine. If dirty, it miscalculates the air volume, causing the computer to inject too much fuel. This is frequently caused by over-oiling aftermarket reusable air filters.
A seized brake caliper piston or sticking slide pin creates constant friction against the rotor, forcing the engine to work much harder to maintain speed. Often caused by failing to grease slide pins during a recent brake job.
An injector that fails to close properly will constantly spray raw fuel into the cylinder. This drastically lowers MPG, causes rough running, and risks severe engine damage from washing oil off the cylinder walls.
Fuel is escaping the vehicle before it can be burned. This is sometimes caused by rusted lines, but is often an installation error where the fuel pump lock ring or gasket wasn't seated properly after a recent fuel pump replacement.