If your vehicle is showing sudden and noticeable drop in fuel economy (bad mpg), here are the most likely causes and the parts you'll need to fix each one.
Low tire pressure drastically increases rolling resistance, forcing the engine to work harder to maintain speed. This is the most frequent cause of a sudden MPG drop, especially during seasonal temperature changes.
A seized brake caliper or stuck slide pin keeps the brake pads pressed against the rotor, creating constant mechanical drag. This frequently occurs if slide pins aren't properly cleaned and lubricated during a recent brake job.
The upstream O2 sensor measures exhaust gases to adjust the fuel mixture. If it degrades and falsely reads a 'lean' condition, the engine computer will compensate by dumping excess fuel into the cylinders.
The MAF sensor measures incoming air to calculate fuel needs. If it gets dirty (often from neglected air filters or over-oiled aftermarket filters), it miscalculates airflow, leading to a rich, fuel-heavy mixture.
If the thermostat is stuck open, the engine never reaches its optimal operating temperature. The computer keeps the engine in a rich 'warm-up' mode indefinitely, consuming significantly more fuel.
Worn spark plugs or weak ignition coils cause incomplete combustion. This means unburned fuel is pumped out the exhaust instead of generating power, requiring more throttle to maintain speed.
A fuel injector that is stuck open or leaking will constantly drip raw fuel into the engine cylinder, destroying fuel economy and potentially damaging the catalytic converter.