For the past 3 months i have had CEL 21 & 21 (O2 sensors).

I just bought a Bosch O2 sensor for $70.00 at autozone, installed it, reset the computer, and the 21's go way.

But CEL code 51 shows up. Can anybody please help me fix this or explain why it happened. It has somthing to do w/ "fuel system lean". I am clueless, please help, selling my car. Thanks

well, if its lean id clean the injectors, change the fuel filter, look for leaks, ect.

i added some fuel injection cleaner, anything else i should try? Thanks

Try making sure your o2 sensor connections are on tight. And read the sensor itself. I think it might mean the o2 sensor is telling the ecu that your car is running lean (low max voltage at WOT). I think at WOT it should be 0.88volts or higher...

how do i test that?

Edit: Oh and now CEL code 21 is back, so i have 21 and 51, but the car is running perfect. Would one of those dummy rear O2 sensors fix the probelem?

Possibly it is just an issue with the fuel trims. Leave it be for a while, then reset the ecu once more and see if code 51 goes away.

21 is probably for the rear o2, since the front one was replaced.