I have an 05 r53 with a 17% sc pulley reduction that was installed by previous owner. Has been throwing lean codes for past few weeks and cannot get to the bottom of it.
Are larger injectors such as 380cc necessary with this size pulley reduction? Any other likely causes of lean code?

 

I run a 15% with stock injectors with no problem. Larger injectors would require a costly tune because your ECU has to be adjusted for a differing injector duty cycle. Have you tried injector cleaner in the gas tank?

 

Lean codes could also arise from a vacuum leak.

 

You don't need bigger injectors. Need to diagnose the fault. It could be many thinks as simple as a vacuum leak, exh leak, bad sensor and so on.

 

Ditto, undersized injectors will not just all of a sudden start throwing a lean code, if anything it would have always been there if that were the case.

And yes, vacuum leaks should be the first thing to look at, our fuel regulators are controlled by vacuum, incorrect vacuum/boost, incorrect fuel delivery to the injectors.
And yes, the ECU does such a good job of controlling things, it doesn't take much to hide a vacuum leak so that it's not really noticeable with the normal rough idle signs.