Poor Fuel Economy: Why Your MPG Dropped
A noticeable drop in fuel economy usually has a mechanical or sensor cause — and the check-engine light often already knows the answer.
Common symptoms
- Measurable drop in MPG over a tank or several tanks
- Check-engine light on with the economy loss
- Rough running, misfires, or power loss alongside
- Fuel smell or rich-running symptoms
What kills fuel economy
Engine management faults top the list: a stuck-open thermostat keeping the engine cold (P0128), a misfire wasting fuel, a rich mixture (P0172), a lazy oxygen sensor skewing fueling — the computer's efficiency depends on accurate inputs. A clogged air filter or dragging brakes add mechanical losses; underinflated tires add rolling resistance.
When economy drops with a check-engine light, the stored code usually names the system responsible. Fixing the fault usually restores the mileage.
Basic checks
Verify tire pressures, check for the obvious (a roof rack left on, extra weight, winter fuel blends), and track mileage over a few tanks to confirm the loss is real rather than one bad measurement.
Professional diagnosis
If the light is on, a code read and proper diagnosis beats guesswork. Shops verify sensor data against expected values and find the specific fault — fuel trims, temperature behavior, and misfire counters tell the story.
Safety first
Related diagnostic trouble codes
These codes are stored alongside this symptom often enough to be worth checking — remember that a code identifies a condition, not a failed part.
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