How do you fix lean air to fuel ratio?

What is wrong if a Neon 96 has very high fuel consumption and the check engine light is showing code 51?

I found this for you: The 51 code means "Fuel too lean." As opposed to a 52, which means "Fuel too rich." Now what does this mean? alldata.com had this to say: Jim Grant's Tech Tips What is Fuel Too Lean? Q: What is "fuel is too lean" mean in the diagnostic test? A: Fuel Too Lean is possibly one of the most difficult computer codes to diagnose. Why is that? There are a couple of things that you need to understand first, before you understand what too lean means. An engine to run correctly needs a good balance of air and fuel. This balance is called the air /fuel ratio. A correct balance of air and fuel is generally at a ratio of 14.7 pounds of air to one 1 pound of fuel. If you change either number of that ratio, is will have an effect on the engine's performance. The computer in your vehicle is always working to maintain the correct air/fuel ratio. However, as time goes on the engine wears, fuel injectors become restricted, vacuum leaks happen and sensors may become worn and/or lie to the computer. After all, it is a rough environment under the hood of your vehicle. The engineers designed the program in your vehicle's computer to make adjustments to compensate for changes that occur with time. This ability of the computer is called adaptive learn. But, there is a limit to how much the computer can adapt, all the while, maintaining the ideal air/fuel ratio. If the computer determines that it cannot adapt any more to provide enough fuel for the amount of air entering the engine it will turn on the check engine light and report a code indicating that the system is too lean. Too much air entering the engine for the amount of fuel available. There is also a code for the opposite condition of the engine receiving too much fuel, system too rich. In either case, the code does not indicate a broken sensor, all it means is that the computer cannot maintain the correct air/fuel ratio and it wants help in fixing the problem. I've seen many parts replaced because of this code that did not fix the problem. You'll need a technician that understands and has the necessary diagnostic tools to provide the correct solution to a system too lean computer code. Good luck.