I recently for my idler pulley changed(the one that feeds the belt) and now my check engine light is on!!! Does anyone know what might have caused this? The car runs and sounds a lot better but the light won't go off.

 

Have you pulled the code?

 

It says p0171 air / fuel mixture lean bank (cylinder number 1 denotes bank)

 

The code is a generic code. Bank1 means cylinders 1-4 not cylinder 1. If it was a v motor and the lean condition was on the other side, it would be p0174 bank2.

I say check vacuum lines, you probably knocked something lose and have a vacuum leak. Get some propane, brake clean or wd40 and start spraying around the engine looking for a rise in idle.

 

^^~~~x2

Vacuum line, maybe you forgot to plug in the MAF? I'm sure it's just a matter of double checking all your connections.

 

Spray some starting fluid around. I there is a hose off/leak you will soon find the engine speed up.