Fuel use went up — where to start
Three readings separate a real fault from winter and traffic.
First rule out winter and traffic
In winter fuel use honestly rises: warm-ups, thicker oil, winter fuel, the heater. Two or three litres per hundred more in frost is not a fault. Compare with the same season last year, not with summer.
Read the fuel trims on a warm engine
This is the key number. Short and long term trims are read TOGETHER. A steady plus means the unit keeps adding fuel: it sees a lean mixture and compensates. That compensation is the extra fuel use.
Check the engine reaches operating temperature
While the unit thinks the engine is cold it runs rich — honestly so. A thermostat stuck open adds a litre or so per hundred all year round. The tell-tale is the same as in winter: a weak heater.
Compare air flow with revs
A dirty air-flow meter under-reads, the unit under-fuels and the driver presses harder — use goes up along with the effort. At idle it is usually 2-5 g/s; noticeably less on a healthy engine is a reason to look at the sensor.
Only then look at mechanical causes
Soft tyres, a blocked air filter, dragging brakes, a loaded boot — all add consumption without changing the trims. If the readings are steady, the fault is not in the fuel side.