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I have a 2003 Ford Gt Mustang 4.6 v-8 engine and the service engine light is on and I had it put on a computer and it read the back passenger side oxygen sensor is bad...it has 4 oxygen sensors..so i switched to 2 back oxygen sensors and had it checked again and it read the front passenger side one was bad so.I switched to 2 back ones back and bought a new one and put it on and it read the new one was bad..so I called 2 diffrent garages and both said it is probley the cataltytic converter..so I put a new one on it but I put the old oxygen sensor back on it..the owners manual said to run 87 octane gas in it andthe man I bought it off of put about 14,ooo miles on it and ran 93 octane in it .you think it might have done somthing to the car to make the light stay on the car runs good and gets good gas mileage.