1995 240SX: Trouble Codes P0130 (Front 02 sensor) & P0325 (Knock Sensor)

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A year ago, I had the front O2 sensor replaced and the Check Engine Light came on a couple weeks ago so it was replaced again by the dealer. The light came back on a week later with the codes P0130 and P0325. I don't know what the codes were in the previous cases.

I don't notice any performance changes. Any ideas on what to do now? Is is worth cleaning the fuel injectors. Other than running rich and getting bad gas mileage, does it need to be fixed?

Thanks,Mark