What Do O2 Sensors Do?
02 sensors measure the amount of oxygen in the exhaust and report the level to your vehicle’s computer. There are O2 sensors before and after the catalytic converter. The upstream O2 sensor controls how much fuel is added to the air/fuel mixture. The downstream O2 sensor makes sure the catalytic converter is working.
If the fuel ratio is lean (not enough fuel in the mixture), the voltage is relatively low — approximately 0.1 volts. If the fuel ratio is rich (too much fuel in the mixture), the voltage is relatively high — approximately 0.9 volts. When the air/fuel mixture is at the stoichiometric ratio (14.7 parts air to 1 part fuel), the oxygen sensor produces 0.45 volts.
Learn the difference between rich and lean fuel mixtures. Proper air fuel ratio is critical to keeping your car running as smoothly as it should be.
Where are Oxygen Sensors Located?
Every car made after 1996 has an O2 sensor upstream and downstream of each catalytic converter. Most vehicles have two O2 sensors. There can be 4 O2 sensors depending on your engine and exhaust. V6 and V8 engines with dual exhaust have four oxygen sensors. A pair of O2 Sensors for each bank of the engine.
Bad O2 Sensor Symptoms
Symptoms of a faulty O2 sensor may include the following:
- Lean or rich running condition
- Poor acceleration
- Engine hesitation
- Black smoke from tailpipe (rich running condition) black smoke is excess fuel exiting the exhaust
- Rough idle
- Vehicle stalling out
- Reduced fuel efficiency
When an O2 sensor fails it will usually trigger your check engine light to turn on. An O2 sensor can fail for many reasons. Use an OBD2 scanner like FIXD to see the specific fault code that triggered the check engine light. Based on this fault code, it will point to how it failed and then move forward with the diagnosis.
You can also use the FIXD Sensor and app to measure the voltage of the O2 sensors and determine if they are faulty and need to be replaced. This will help you understand the root cause of your problem.
ProblemSymptomsFault Code Upstream O2 sensor circuit malfunctionDecrease in MPG, Increased tailpipe emissions, asymptomatic sometimesP0130 Low upstream O2 sensor voltageLack of power, Vehicle running rough, Exhaust odor, asymptomatic sometimes P0131 High upstream O2 sensor voltageVehicle running rough, Lower fuel economyP0132 Downstream O2 sensor circuit malfunctionDecrease in MPG, Increased tailpipe emissions, asymptomatic sometimesP0136 Low downstream O2 sensor voltageLack of power, Vehicle running rough, Exhaust odor, asymptomatic sometimesP0137 High downstream O2 sensor voltageLower fuel economy, rough idle, strong fumesP0138 Faulty oxygen sensor (lean)Lack of power from the engine, rough idle, engine coughing/misfiring, tips of spark plugs are whiteP0171 Faulty oxygen sensor (rich)Lack of power from the engine, rough idle, engine hesitation/misfiring, soot buildup on spark plug diodeP0172 Faulty catalytic converter (possibly due to broken O2 sensor)A lack of power from the engine, decreased fuel efficiency, rotten egg or sulfur smellP0420
O2 Sensor Voltage Range
Sensor NameLocationVoltage RangeReasons For Being Out Of Range Upstream O2 Sensor/Bank 1 Sensor 1Between the exhaust manifold and the catalytic converterThe voltage should fluctuate between 0.1V and 0.9VIf the voltage never exceeds 0.75V, the air/fuel mixture is lean If the voltage never drops below 0.25, the air/fuel mixture is rich If the voltage is steady, there is probably a problem with the sensor itself Downstream O2 Sensor/Bank 1 Sensor 2Between the catalytic converter and the mufflerThe voltage should be steady at about 0.45VIf the voltage is steady and high there is a lack of oxygen in the air/fuel mixture If the voltage is steady and low, there is too little oxygen in the air/fuel mixture If the voltage is fluctuating like the upstream O2 sensor, the catalytic converter may be broken