Symptom guide
Poor Fuel Economy Suddenly
Common causes when a car starts using noticeably more fuel than usual.
Quick answer
Sudden poor fuel economy can come from rich mixture, oxygen sensor data, dragging brakes, low tire pressure, thermostat issues, fuel leaks, or driving conditions.
Most likely causes
- Rich fuel mixture
- Oxygen sensor problem
- Low tire pressure
- Dragging brake
- Thermostat stuck open
Basic driver checks
- Check tire pressure.
- Look for fuel smell or leaks.
- Scan for fuel trim, oxygen sensor, thermostat, or misfire codes.
- Notice if the engine takes longer to warm up.
DIY diagnostic path
- Do not replace oxygen sensors based only on mileage.
- Compare fuel use over several fill-ups.
- Check for roof racks or load changes.
- Investigate fuel smell promptly.
Common mistakes
- Blaming fuel quality without checking codes
- Ignoring dragging brake heat
- Replacing parts before checking basic tire pressure
What to tell the mechanic
Provide recent MPG change, driving pattern, codes, tire pressure, and any smell or smoke.