If your vehicle is showing noticeable and sudden drop in fuel economy, here are the most likely causes and the parts you'll need to fix each one.
A failing upstream O2 sensor often falsely reads a 'lean' condition, causing the engine computer to continuously dump excess fuel into the cylinders to compensate.
A dirty MAF sensor miscalculates the amount of air entering the engine, leading to an overly rich air-fuel mixture. This is frequently caused by over-oiling aftermarket reusable air filters during maintenance.
Worn spark plugs or failing ignition coils cause incomplete combustion, meaning unburned fuel is wasted out the exhaust instead of generating power.
A seized brake caliper or stuck slide pin keeps the brake pads pressed against the rotor, creating constant mechanical resistance that the engine must burn extra fuel to overcome. Often caused by failing to lubricate slide pins during a recent brake job.
If the thermostat is stuck open, the engine never reaches its optimal operating temperature. The computer assumes the engine is still warming up and stays in a fuel-heavy 'open loop' mode.
While modern computers compensate for airflow, a severely restricted air filter forces the engine to work harder to breathe, reducing power output and causing the driver to use heavier throttle to maintain speed.
A fuel injector that fails to close completely will constantly drip raw fuel into the intake or cylinder, drastically reducing fuel economy and potentially causing internal engine damage by washing oil off the cylinder walls.
A restricted catalytic converter chokes the engine's exhaust flow. The engine has to work significantly harder to push exhaust gases out, requiring much more throttle and fuel to achieve normal speeds.