Fuel Economy Suddenly Dropped — Where to Look First

What's happening

Fuel economy is sensitive to small changes. The biggest sudden-MPG-loss culprits are: an aged O2 sensor making the ECM run rich, a stuck-open thermostat keeping the engine cold (forcing open-loop fueling), low tire pressure raising rolling resistance, and a dragging brake caliper. None require a shop most of the time.

You might also notice

• No drivability change — just trips to the gas station
• Possibly a check engine light (P0420, P0128, P0133)
• A wheel that runs hot at the rim after a drive

Likely causes (most common first)

1. Aged oxygen sensor (replace at 80–100k miles regardless of codes)
2. Thermostat stuck open (engine never warms up — P0128)
3. Low tire pressure (often missed — check all four with a gauge)
4. Dragging brake caliper
5. Dirty MAF sensor
6. Leaking fuel injector
7. Different fuel grade or seasonal blend

What to check first

1. Tire pressure first — free, takes 5 minutes
2. After a 30-minute drive, briefly touch each wheel rim — a hot one suggests a dragging caliper
3. Watch the temp gauge — does it read low? Stuck-open thermostat
4. Read codes — fuel-trim codes will point at MAF or O2

Common OBD2 codes for this symptom

Don't have the code yet? Look up your code or read it with AXLY.pro.

Can I keep driving?

Yes — but the longer you wait, the more you spend on gas.

Confirm with the actual code

Symptom-based diagnosis narrows the field — reading the actual stored code finishes the job. AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter and reads every stored DTC.