Code 21 - Probable Bad Oxygen Sensor

[jbreyes]

My 1997 Dodge Grand Caravan 2.4L with a 21,000 mileage just had the Service Engine Soon light on. When i checked out what error code it has, it showed the Code 12 and 21 errors.

I looked for it under allpar.com and it says that CODE 12 means battery or computer recently disconnected(not really a problem) while CODE 21 means Oxygen Sensor signal doesn't change. Probably bad oxygen sensor.

I don't experience any problems at this point. Is it that accurate or I can also try and reset the computer by pulling off the negative ground on the battery for about 15 seconds?

If I need to replace the oxygen sensors, do I replace both? What will happen if I delay the replacement?

What symptoms will I see if I have this problem? I have seen some comments like increase in fuel consumption. What causes this type of problem, fuel used? As I have said, I am not exprienceing any problems with my engine at this time.