The MIL light is on, after being given the 54 code, and advice to replace the sensor, we replaced the downstream sensor with an OEM part. The light still comes back on after only a few miles. After re-checking with an OBD scanner, we're getting the same code, but now are being told it's something between the sensor and the computer...not the actual sensor itself. The fuses are fine, the lower sensor is new and fine, what would be the next logical thing to check to get this darn light to stay off?? My daughter just bought this car and can't get it registered without emissions. (will they even check emissions with the light on?) Any help is appreciated.