* Oxygen Sensor (O2 Sensor) Malfunction: This is a very common culprit. The O2 sensor monitors the air-fuel mixture. A faulty sensor provides inaccurate data to the engine control unit (ECU), leading to a richer (more fuel) or leaner (less fuel) mixture than optimal. A richer mixture will burn more fuel without a noticeable power increase, explaining both symptoms.
* Mass Airflow Sensor (MAF Sensor) Issue: The MAF sensor measures the amount of air entering the engine. A faulty sensor can provide incorrect readings, causing the ECU to adjust the fuel mixture incorrectly, again resulting in excess fuel consumption.
* Vacuum Leak: A leak in the intake system can allow unmetered air to enter the engine, throwing off the air-fuel ratio. This will typically result in increased fuel consumption and potentially a rough idle, though the symptoms can be subtle.
* Catalytic Converter Issues: A failing catalytic converter can restrict exhaust flow, potentially leading to increased fuel consumption. This is less likely to be the primary cause if the car runs fine otherwise, but it's possible.
* Fuel Injector Problem: A clogged or leaking fuel injector can cause an imbalance in fuel delivery, leading to inefficient combustion and increased fuel consumption.
* Throttle Position Sensor (TPS) Problem: A faulty TPS provides inaccurate information about the throttle position to the ECU, affecting fuel delivery.
* PCV Valve (Positive Crankcase Ventilation) Issues: A faulty PCV valve can cause excess pressure in the crankcase, leading to oil leaks and affecting fuel efficiency.
What to do:
1. Get the diagnostic trouble codes (DTCs) read: The check engine light stores codes that pinpoint the likely problem(s). You can do this with an OBD-II scanner (available relatively inexpensively at auto parts stores). The codes will give you a much more precise direction.
2. Have a mechanic inspect the car: Based on the DTCs or even without them, a qualified mechanic can diagnose the specific issue. They can perform tests to check the components listed above. Ignoring the problem can lead to further damage and more expensive repairs down the line.
Don't delay getting this checked. Continued driving with increased fuel consumption is costly and potentially harmful to your engine.
