If your vehicle is showing sudden drop in fuel economy or bad gas mileage, here are the most likely causes and the parts you'll need to fix each one.
Low tire pressure increases rolling resistance, forcing the engine to work harder and burn significantly more fuel to maintain speed.
A severely dirty air filter restricts airflow into the engine, which can alter the air-fuel mixture and reduce overall engine efficiency.
Old spark plugs or failing ignition coils cause incomplete combustion, meaning unburned fuel is wasted out the exhaust instead of generating power.
O2 sensors monitor exhaust gases to adjust the fuel mixture. A failing or lazy sensor often defaults to a 'rich' condition, dumping excess fuel into the engine.
A stuck brake caliper or seized slide pin keeps the brake pads pressed against the rotor, creating constant drag that destroys fuel economy and overheats the brakes.
If the thermostat is stuck open, the engine never reaches optimal operating temperature. The computer thinks the engine is always cold and continuously injects extra fuel to warm it up.
The MAF sensor measures incoming air. If dirty (often from over-oiled aftermarket filters), it miscalculates airflow and causes the engine to run rich.
A restricted catalytic converter chokes the engine's exhaust flow, drastically reducing power and fuel economy as the engine struggles to push exhaust gases out.