Sudden bad gas mileage — what live data actually shows

Fuel economy that falls overnight is a diagnostic clue. Economy that was always poor is a driving-style or tire-pressure conversation. Start by asking: did a light come on, did you recently “clean” something, or did the weather change?

MPG screens in cars and apps are estimates. Use them as a trend, then confirm with fuel trims, coolant temp, and MAF — not with a single tank.

The four common OBD2 stories

Running rich

P0172 / P0175, or strongly negative long-term fuel trim. The ECU is pulling fuel and still cannot get the mixture right — leaking injector, high fuel pressure, or a lying MAF/O2. Watch STFT/LTFT at idle and at a steady 50–60 mph cruise. They should not both sit at −15% or worse.

Engine never fully warms up

P0128 (coolant thermostat) is a classic mileage killer. If coolant temperature hangs at 70–80°C on the highway, the ECU keeps extra fuel. That is a thermostat / cooling-system diagnosis, not a “tune-up.”

Airflow the ECU does not believe

P0101 and related MAF codes, or a MAF that reads too high for the RPM. Extra reported air → extra fuel. Compare MAF g/s to RPM and load. After a reconnect or a dirty filter, numbers can look wrong until you verify the intake is sealed.

The light is on for something that wastes fuel

A misfire (P0300) dumps fuel into the exhaust. A stuck-open thermostat or a rich O2 bias does the same more quietly. P0420 itself does not always raise fuel use — the misfire that caused it does.

Non-OBD things that still feel like “the car got thirsty”

• Under-inflated tires, a dragging brake, or a roof box
• Short winter trips only (the engine never reaches closed loop for long)
• A different fuel blend or a lot of idle / AC

If trims, coolant, and MAF look normal and there is no code, measure those mechanical items before replacing sensors.

A 15-minute MotorBit check

1. Read codes and freeze frame.
2. At a hot idle: STFT, LTFT, coolant, MAF, RPM.
3. On a flat road at a steady speed: same PIDs. Note whether trims move a lot or sit pegged.
4. If coolant never reaches normal, stop guessing injectors.