I'm currently driving a 2000 Volvo S40 T4, and have realized that within the last 2 months my fuel/gas consumption has increased dramatically. I would appreciate advise on what to do and what the cause may be as it has not been in for servicing that it is due for at the workshop.

1. Check for Diagnostic Trouble Codes.

2. Replace the Oxygen Sensor. A bad oxygen sensor can radically affect gas mileage, since it controls fuel injector duration. It's best to use the exact brand that came in the vehicle.

3. Go over the whole ignition system. Replace, for example, spark plugs (with exact brand and plug number that came in the vehicle), spark plug wires, rotor, and distributor cap. Replace platinum plugs at half the recommended mileage.

4. Replace air filter and fuel filter.

5. Check idle rpm and engine timing.

6. Clean MAF Sensor, if so equipped, with CRC MAF Sensor Cleaner. Gas mileage and performance can improve markedly.

7. Replace the Engine Coolant Temperature (ECT) Sensor, which affects fuel injector pulse width. The ECT Sensor is a temperature dependent resistor (thermistor), whose resistance decreases as the engine coolant temperature increases. It provides a richer mixture when cold, leaner mixture when hot. They become corroded, due to not changing the coolant frequently enough, flood the engine, and waste a tremendous amount of fuel.

8. Check tire pressure.

9. Add Shell Regane or Chevron Techron fuel injector cleaner to gas tank every 6 months. This will clean fuel injectors, throttle body, valves, and combustion chamber.

10. Use full synthetic lubricants; e.g. engine oil, manual synchromesh transmission fluid, automatic transmission fluid, and differential fluid. This should increase gas mileage by 8 to 10%.

11. Check for dragging brakes, including emergency brake.

12. Check alignment.