There are two types of oxygen sensor Narrow band and wide band the narrow band will be four wires or less while the wideband most of the time will be more than four wires today the narrow band specifically the zirconia type

Producing its own Voltage :

 The purpose of the oxygen sensor is to measure the amount of oxygen in the exhaust system and communicates it with the e cm it does this by producing its own voltage by measuring the difference between oxygen and the oxygen exhaust system versus the oxygen in the atmosphere it only produce a voltage when the sensor is heated up to 600Fahrenheit but for faster and more efficient results it needs to be at 1500 Fahrenheit most cars have  two oxygen sensors one before the catalytic converter and one  after some have three or even  four depending on your engine size 0r 2 catalytic converters the oxygen sensor your engine size or if you have two catalytic converters the oxygen sensor before the catalytic converter is used by the e cm  to add or reduce fuel by controlling the fuel injectors to achieve the perfect air fuel ratio

More fuel Less Air, More Air less Fuel :

For Example :

If the oxygen sensor measures a rich condition which means more fuel and less air the e cm will reduce fuel if the sensor measures a lean condition which mean more air and less fuel the e cm will command more fuel on the other hand the action sensor after the cat is used to calculate the efficiency of the catalytic converter now there are four types of zirconia oxygen sensors we will start things off with the first task that we applied to all four types and is only for the oxygen sensor before the catalytic converter you will need a digital multimeter In first step is to ensure that your vehicle engine is turned off before starting off before starting to test an oxygen sensor position your multi meter to 2 volts on the dc voltage section using a back probe test lead connect the red lead of the multi meter to the signal wire of the oxygen sensor then touch the black lead to a solid ground this can be the negative side of the battery or simply by touching the metal frame of the car now start your engine and wait until the oxygen sensor heats up to at least 600 Fahrenheit to be able to produce a voltage reading and allow the car to go into closed loop once it is you want your voltage reading to fluctuate below 300mV to over 800milli volts or below 0.3 to over 0.8 volts if this is the case your oxygen sensor should be good and does not need  to be replaced if you oxygen sensor is malfunctioning and needs to be replaced

WON,T FLUCTUATE:

If wo not fluctuate at all or it will stay close to 450milli volts or 0.45V at all times now  if the o2 sensor reads higher than 550 millivolt at all time this indicates a rich air  fuel mixture and could be caused by one of the following the high reading could also be caused by the sensor giving a bad reading due to contamination if you have a reading that’s below 350mv at all times this indicates a lean air fuel mixture and could be caused by defective oxygen sensor or one of the following an easy way to check if  it’s the o2 sensor is the floor on the gas pedal  you should see the o2 sensor react instantly and increase voltage if not then most likely the o2 sensor  is faulty the second test only applies to the auction sensors with the heater and can be used for both sensors before and after the cat first make sure your vehicle is off you want to set your multi meter to 200 in the ohms  position to measure resistance next dis connect the oxygen sensor and connect one test lead to heater terminal while connecting the other lead to the second heater terminal on most cars the heater wires will be the same color but make sure you look into your vehicle repair manual if you don’t have if you get no readings at all then you have a bad heater and the sensor needs to be replaced if you do have a reading then that’s a good sign just make sure its within specification according to your vehicles repair manual I have seen specification being from 2 to 4 5 to 7 and even up to  16 ohms now other than the auction sensor being bad make sure your wires are not damaged and the connectors are free of corrosion as these can make your oxygen sensors malfunction good deal .

RICH CONDITION :

(MORE FUEL ,LESS AIR)

• Leaking fuel injectors
• MAF sensor
• ECT
• fuel pressure Regulator
• purge valve

LEAN CONDITION:

(MORE AIR,LESS FUEL)

• Vacuum leak
• Clogged fuel filter
• Fuel pump
• MAF sensor
• Clogged fuel injectors
• Torn brake boosted diaphragm