I have a 97 dodge ram 1500 5.2 L.

I have this code that keeps coming up: Code 51. I know it means that my engine is running lean.
I took it to the mechanic and he cleaned out my injectors and fixed a leak in my exhaust. Well the light came back on.

Can there be any other problem that cam cause this code that may be easily overlooked? Please be specific; I plan on using the info to fix it myself.

Thank you all in advance for any help.