Why Is My Car Using So Much Fuel?

If your fuel economy has dropped noticeably — say, 20% or more below the manufacturer's quoted figures — something is wrong. Modern engines are precisely calibrated, and any fault in the fuel, air, ignition, or emissions system can cause excessive fuel consumption.

Before blaming the car, rule out driving habits: heavy traffic, short trips, aggressive acceleration, under-inflated tyres, and roof racks all increase fuel use significantly.

Common Causes of High Fuel Consumption

Mechanical and sensor-related causes:

• Faulty oxygen sensor — The ECU relies on O2 sensors for fuel trim. A lazy or failed sensor causes rich running.
• Dirty air filter — Restricts airflow, forcing richer fuel mixture.
• Worn spark plugs — Incomplete combustion wastes fuel.
• Faulty thermostat stuck open — Engine never reaches operating temperature, runs in cold-start enrichment mode.
• Dragging brakes — A stuck calliper wastes energy through friction.
• Low tyre pressure — Under-inflated tyres increase rolling resistance.
• Faulty MAF sensor — Incorrect airflow reading causes wrong fuel calculations.
• Fuel injector leak — Dripping injectors waste fuel and wash oil from cylinder walls.

Related OBD-II Codes

These codes often correlate with poor fuel economy:

• P0171/P0174 — System Too Lean (causes the ECU to add extra fuel)
• P0172/P0175 — System Too Rich
• P0128 — Coolant Thermostat Below Regulating Temperature
• P0420 — Catalyst System Efficiency Below Threshold
• P0101 — MAF Sensor Range/Performance
• P0130-P0167 — Various Oxygen Sensor codes
• P0300-P0312 — Misfire codes

How to Improve Your Fuel Economy

Start with free and cheap fixes:

1. Check tyre pressure — Inflate to the manufacturer's recommendation on the door jamb sticker.
2. Replace the air filter — A $30 filter can improve efficiency by 5-10%.
3. Scan for fault codes — Even without a check engine light, pending codes may indicate issues.
4. Check fuel trims — Use MyMait to monitor short-term and long-term fuel trims. Values outside +/-10% indicate a problem.
5. Service the engine — Fresh spark plugs, oil change, and clean throttle body.
6. Check the thermostat — If the engine takes a long time to warm up, the thermostat may be stuck open.
7. Inspect brakes — Jack up each wheel and spin it. It should spin freely.

Estimated Repair Costs

Australian cost estimates for common fuel economy fixes:

• Air filter: $20–$60
• Spark plugs: $80–$250
• Oxygen sensor: $150–$350
• Thermostat replacement: $150–$350
• MAF sensor: $150–$400
• Brake calliper service: $100–$300 per wheel
• Full engine tune-up: $200–$500