Why is my car using too much fuel?
A sudden drop in fuel economy (MPG) is nearly always a sign that something is wrong with the engine or exhaust management system. Common culprits are a faulty oxygen sensor (which causes the engine to run rich), a failing thermostat (which keeps the engine too cold), or dirty fuel injectors. Tyre pressures, wheel alignment, and driving style also affect economy, but a dramatic drop points to a mechanical cause. The engine warning light is often on at the same time.
Likely causes
Faulty oxygen / lambda sensor
O2 sensor tells the ECU how much oxygen is in the exhaust; a failed sensor causes the engine to run rich (too much fuel)
Failing thermostat (stuck open)
Engine never reaches full operating temperature; ECU runs rich warm-up strategy continuously
Dirty or leaking fuel injectors
Injectors that drip or spray poorly cause excess fuel to enter the combustion chamber
Air filter blocked
Restricted airflow forces the engine to use more fuel — quick and cheap to check
Low tyre pressure
Under-inflated tyres increase rolling resistance; check pressures against door sticker spec
Dragging brakes or sticky caliper
A seized brake caliper creates constant friction, increasing fuel consumption and wearing the tyre unevenly
Repairs you may need
Typical cost range: £40–£350 — O2 sensor: £80–200. Thermostat: £80–200. Air filter: £20–60. Fuel injector service: £80–200.
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