2007 GMC C1500 Engine codes PO171 & PO174

I had the 60k service done at 66k and the engine light came on the next week. The mechanic at the Tire Barn said it was either the fuel pump or the catalytic converter, then turned the code off. Since I drove it in without problem he said to bring it back when they come on.
When the engine light came on again he said it was the catalytic converter and to take it to the dealer since it was under warrenty till 80k. The dealer first had to run their own diags and said it was dirt in the injectors and wanted 139.99 to clean them. I told them I just had it serviced. I called the Tire Barn and the mechanic said he would re-do it at no charge.
When the engine light came on again the Tire Barn mechanic said without any problems in drivability it is hard to determine whether to change the fuel pump or the converter, or even clean the o2 sensors.
Do you have any info that will help solve this problem