11. Oxygen sensor, or lambda sensor, fuel injection system.

The oxygen sensor (also called a lambda sensor) looks like a spark plug screwed into your catalytic converter. It's actually an important part of your fuel injection system. The oxygen sensor continually tests the level of oxygen in the exhaust stream, and feeds this information back to the fuel injection computer. Too much oxygen means the engine is running lean and the computer needs to send more fuel to the cylinders.  Too little oxygen means the engine is running rich and the computer needs to send less fuel.  This fine-tuning procedure goes on constantly to ensure the proper air/fuel ratio.

A bad oxygen sensor will continually tell the computer that the engine is too lean, and the computer will respond by sending more fuel. This is bad, not only for fuel economy and emissions, but a rich mixture will eventually burn out your catalytic converters, possibly causing a fuel fire inside the exhaust system, which can then spread to the rest of your car. It's a bad thing.

According to Jaguar and the other auto manufacturers, the oxygen sensor should be replaced every 30,000 miles to ensure troublefree performance.

There are two different types of oxygen sensors, sensors with one wire and sensors with three wires. The three wire sensors have a built-in heating element that brings the sensor up to operating temperature quickly. When you replace the sensor, be sure to get the correct type for your car. If your Jag is less than 5 years old, you may be able to get the oxygen sensor replaced under warranty, even if the warranty for the rest of the car has expired. It's worth checking out. The V12-equipped cars have two oxygen sensors, while the six-cylinder cars only have one.

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