This CEL lite started shortly after having our gas tank and sender replaced. They also replaced the flex pipe. A few days later the light CEL came on with that code. Fuel setting rich according to the code chart. I replaced the MAP sensor and front O2 sensor but the indication still comes back. Can it be the rear O2 sensor? What else can I check? It is a lot harder on gas since then and the exhaust smells stronger as well while warming up. It starts ok and runs good but the wretched light is annoying. Help please?

Phil