Here's one explanation of the 51 code, from right here on good old Allpar:
- 51 Oxygen sensor stuck at lean position (Bob Lincoln wrote: may be tripped by a bad MAP sensor system causing a rich condition, and the O2 sensor trying to compensate. The O2 sensor may still be good. The MAP assembly consists of two pieces, the valve and the vacuum transducer (round plastic unit with cylinder on top and both electrical and vacuum connections) - If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor. [Webmaster note: MAP sensors seem to die regularly.]
So basically, the computer is being told that the O2 sensor is STUCK and telling the computer that it (the sensor) is reading a lean condition in the fuel-air exhaust mix. That means the O2 sensor is faulty. It may be faulty because some other part has failed (i.e., MAP sensor), nonetheless it is faulty. Replace it. Trying to "clean" it will leave some residue on the reactive surface of the sensor, and it will send codes to the computer again. Replace it. You may also need to repair or repalce other parts; that will require more diagnosis, but a bad O2 sensor does have an immediate and irreconcilable negative impact on gas mileage, until it is replaced.
You will be amazed at the immediate improvement - been there a couple of times with my '92.