2002 F-150 crazy codes

On a 2002 ford F150 4.2 6 cyl. I have a service engine soon light that keeps coming on. Pulled the codes they were 0171 and0174. Took it to firestone, they replaced the intake manifold gasket, ran fuel system cleaner and replace the pvc tube. Truck has always ran like a top. Ran fine no codes for 2 days, light came back on. Firestone then replaced both downstream o2s, again ran fine 2 days light back on. Now theres a slight ticking sound and 2 new codes 1131and 1151. (The first two have always poped when we pulled them). What could be causing this aggrivating problem? It's driving me crazy! Truck still running like a top!