Oxygen sensor terminology can be confusing. Here’s a guide to deciphering it all.


From: garysautotechclub (Yahoo Group) , Author : overtork187


Oxygen sensors are described as “upstream” or “downstream.”
An “upstream” sensor is located near the engine, typically in the
exhaust manifold. A “downstream” sensor is located near the catalytic
converter. Though both perform the same function – measuring the
proportion of unburned fuel and oxygen in the exhaust – the differing
data points allow the engine computer to determine whether all of the
components in the engine management and emissions systems are
properly functioning.

That means, of course, that there will always be at least two O2
sensors in any vehicle – at least those built after 1996, when the
emissions regulations requiring oxygen sensors became law.
Usually, however, there are more – often as many as four.
Any car with a V engine (V-6, V-8, etc.) will have two upstream
sensors, one for each cylinder bank. These are called the “bank 1”
and “bank 2” sensors. Bank 1 is whichever cylinder bank has the
number one cylinder. That’s the one that fires first in the firing
order. (All cylinders on an in-line engine are bank 1.)
The number of downstream sensors varies between manufacturers and
vehicles. One will always be located downstream of the catalytic
converter, so that there is a measure of the converter’s efficiency.
But many car makers use two downstream sensors, one somewhat before
of the catalytic converter and the other after it.

Sensors are also described by the number of wires employed: three,
four, or five. Because O2 sensors don’t function properly until they
reach an operating temperature of about 600ºF., modern practice is to
electrically heat the sensor so that it functions as soon as the
engine starts. How that is accomplished determines whether three or
four wires are used. The latest technology in O2 sensors,
the “wideband” sensor, uses five wires. You must replace a sensor
with one that uses the same number of wires as the original.
Sensors are either “universal” or “direct fit.” Universal sensors are
designed to fit multiple vehicles using the same sensor “bulb,” the
part that sticks into the exhaust. But, universal sensors do not come
ready to install. The electrical connector will have to be assembled
first. In contrast, a direct fit sensor is ready to install right out
of the box and simply plugs into the existing wiring.

Which sensor do you replace?

A scan tool or code reader will retrieve the diagnostic trouble code
set as the result of an O2 sensor malfunction. Trouble codes are
specific to the sensor, so the trouble code will effectively
designate the sensor which set the code.


Check out this Obd2 Scan Tool for retrieving codes

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