How do I check the sensor for proper working order,first it is a 1990 420sel the car began to sputter and failed to take the fuel I unplugged the 02 sensor and it ran fine for 20 miles then started to act up again, did not help when I replugged the sensor. I need to know what the proper numerical value is when testing the o2. the other problem was that when I took readings at the 2&3 pins it would read 50.3 and steadily drop to 0,rev. the eng. and it jumps to 50.3 then back to 0(slow steady drop.)