what is an O2 sensor, and what happens when it fails
an O2 sensor, or oxygen sensor, measures how much oxygen is left in your exhaust. that one reading tells the engine computer whether it is putting in too much fuel or too little, and the computer adjusts the mixture accordingly, several times a second, the whole time you are driving.
get it wrong in one direction and you waste fuel. get it wrong in the other and you damage things.
where it is and how many you have
the sensors screw into the exhaust. the upstream one sits before the catalytic converter and does the fuel trimming. the downstream one sits after it and exists to check the converter is doing its job.
most cars from the late 1990s onward have at least two. a V6 or V8 usually has four, because each bank of cylinders gets its own pair. when a shop quotes you for "an oxygen sensor" it is worth asking which one, because the answer changes the price.
what a failing O2 sensor feels like
it rarely fails all at once. it gets lazy, and the symptoms creep:
- fuel economy drops and you cannot account for it. this is usually the first sign and often the only one for a while.
- the check engine light comes on, frequently with a code in the P0130 to P0167 range, or P0420 when the downstream sensor is reporting on the converter.
- rough idle or hesitation, particularly when the engine is cold.
- it fails an emissions test, sometimes with no other symptom at all.
why replacing it promptly matters
an engine running rich puts unburned fuel into the exhaust, and unburned fuel in the exhaust destroys catalytic converters. a converter costs several times what a sensor costs.
that is the actual argument for not driving on a known bad O2 sensor. it is not that the car will stop. it is that a cheap part left alone takes an expensive one with it.
how long they last
most last somewhere between 100,000 and 250,000 kilometres depending on the sensor and what has passed through the exhaust over its life. oil burning, coolant leaks and a lot of short trips all shorten it.
they are also, quietly, a wear item on any high mileage vehicle. if your car has 200,000 kilometres and its fuel economy has drifted, the sensors are worth testing before you look for anything more complicated.
getting the diagnosis right
a code that names an oxygen sensor does not always mean the sensor is bad. an exhaust leak upstream lets air in and makes a perfectly good sensor report a lean condition. a vacuum leak does the same. so does a failing fuel pump.
replacing the sensor in those cases fixes nothing and costs you a part.
at Arrow Motors in Aylmer we test before we replace, and if the code is pointing at something the sensor is only reporting on, that is what you get told.