P0172

01 grand prix with 3.8l non-supercharge 130k

I've got rough idle and sometimes cranking during ignition in the last several months. Last week, the service engine light was on. I went to autozone to read the code and it was P0172. I just used a bottle of fuel injector cleaner and now the idle is much more stable. But still the engine speed sometimes drop to very low value and jump back to around 1000 when I release the gas pedal. And sometimes engine cranks during ignition especially when the engine is still warm.
BTW, can anyone recommend an OBD II code scanner? Thanks a lot

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)