I read my ECU and it came back with the 0304 code, the knock sensor/circuit. What exactly is the knock sensor? Lately iv been getting really bad gas mileage and read on a few posts thats what other ppl have had also. my car doesn't idle weird or anything. last tiem i checked the ecu it had some error codes but i didn't write them down and they hadn't come back yet but the 0304 kept coming back. What could correct the problem or what could be causing it?