While the onboard diagnostics systems pioneered during the 1980s performed admirably for their time, computer diagnosis really only came into their own with the introduction of the standardized Onboard Diagnostics, Series Two in 1996. OBD-II monitors almost every aspect of engine performance, with the most important being crankshaft position, throttle position and exhaust gas temperature. A "lean engine" code can prove disastrous if left unaddressed, so you need to deal with it as quickly as possible.
All chemical reactions have a limiting reactant, or a reactant that runs out before the others. In an engine, oxygen is the limiting reactant to combustion, since air is a lot more difficult to get into the engine than fuel. Manufacturers typically inject more fuel than the available oxygen can burn because the unburned fuel helps to keep combustion chamber temperatures down. If fuel becomes the limiting reactant, then temperatures inside the engine skyrocket and the entire system stands in danger of melting.
Oxygen sensors are available in several types, but most vehicles use a Lambda-sensor design. The O2 sensor has three parts: a conductive electrical plate exposed to the exhaust, a second plate exposed to the outside air and a piece of zirconium dioxide sandwiched between them.
Zirconium dioxide (also known in a slightly different form as cubic zirconia) acts as a sort of chemical generator; when one side of the zirconia gets hotter than the other, it releases energy in the form of an electrical voltage. The computer reads this voltage and uses it to extrapolate the amount of heat (and thus oxygen) present in the exhaust stream.
A lean condition is always the result of a deficit in fuel (clogged injectors or filter, low fuel pressure), an excess amount of air (generally from a vacuum leak) or a bad oxygen sensor. If the engine runs fine otherwise, then odds are that the O2 sensor has gone bad and needs replacing. This is especially true if the lean code comes hand-in-hand with a drop in horsepower and a loss in fuel economy. When the computer stops trusting the O2 sensor, it goes into an "open loop" computer program that injects more fuel to keep the engine from melting down.
