I have been getting pretty poor gas mileage lately (12-15 mpg) so I was all set to replace my O2 sensor today as that seems to be the cause of a lot of everyone else's problems. Before I got into the engine, I decided to rum my ECU codes and I followed the instructions in the Hayne's manual. Before I replaced the sensor, the computer would flash green, stay lit, flash red, and then both lights would go out. This would repeat over and over. As this varied from the description in the manual, I set to replace the sensor. Now that the sensor is new, the computer is still flashing the same lights. What's up?