History:

2000 RX300 I replaced two 02 sensors (B1S1 and B2S1). I pulled the 20 amp EFI fuse and the 7.5 ECU-B fuse. After a 10 min wait I replaced the 7.5 amp fuse first, then plugged the 20 amp EFI fuse and I immediately heard low buzzing noise coming from the engine bay. It sounded like a fuel pump priming type of noise. It went on for about 30 seconds and didn't seem to want to stop so I pulled the fuse out. Waited a minute and put the fuse back in (oddly enough I turned the fuse around and inserted it the opposite of the first time I put it back in) and no noise this time.

Any ideas on what this could have been? No Check engine light on after a drive so I think everything is ok but not sure what the noise was other than fuel priming I don't know.

Looking for feedbaack,
Thanks
B

 

Well, it wasn't the fuse direction (unless you believe that high-end audio scam of directional speaker cables). it was probably the timimg of firing things up the first time. The fuel pump is in the tank at the back of the car. It's always possible that the pulsations were somehow transmitted to the engine bay.

 

The EFI, and fuel pump relays might have burnt contacts. Pull them and pry the top cover off. Look at the contacts under the center of the coil for arc burns. Also a bad relay will likely have smoke film under the cover.