Fuel consumption suddenly went up
MPG dropped 20-40%, no check-engine light — what to look at through OBD-II.
A sudden MPG drop without visible issues is almost always about fuel trim. The ECU tries to maintain stoichiometric AFR (14.7:1 for gasoline) and adds fuel when some sensor lies to it.
Top 3 causes: (1) dirty MAF sensor under-reports airflow → ECU thinks less air is needed → adds fuel, (2) slow-responding O2 sensor (100k+ miles) → ECU can't close the loop fast enough, (3) thermostat stuck open → engine stays in warmup-enrichment forever.
What to read: LTFT — if it holds ~0% at warm idle, fuel trim is not the culprit. LTFT > +5% → lean correction (suspect MAF/O2). LTFT < −5% → rich (dead injector or IAT sensor).
First diagnostic steps
- Read LTFT/STFT (Mode 01 PID 07/06/09/08)
- Check coolant temp when warm (should be 185-205°F / 85-95°C)
- Visually inspect MAF sensor for dirt (oil residue from PCV is a classic)
- Compare upstream vs downstream O2 signals when warm
- Test fuel rail pressure (spec varies, ~43-51 PSI on most port-injected cars)
Likely fault codes
Click a code for detailed description, causes, and diagnostic plan.