Code PO172

I am getting an error code of PO 172, System to Rich ( Bank 1 ) What will cause this? How do I repair it?
I have a 2001 Grand Prix GT ,3.8 with 107,000 miles.

DTC P0172 indicates the system is in a rich trim mode. Ideal fuel trim = zero. Greater than zero indicates the PCM is adding fuel to compensate for a lean condition (P0171). If fuel trim is less than zero the PCM is reducing the amount of fuel to compensate for a rich condition.

Here are possible causes:

Check all vacuum hoses at intake system.
PCV Valve
Possible throttle body blockage (clean input screen and body)
IAC (idle air controller valve)
Check fuel injectors (bad o-ring seal)
MAF sensor
EGR valve
Check 02 sensor (before CAT)