First the answer to your second question. "Generally" the stated life of the O2 sensor is 100K. When one goes bad, the 'check engine light' should come on and the diagnostics from that should tell you which one is bad. If your vehicle has at least 100K, then you get into the "might as well as" area-if one is bad, could the others be far behind?
Replacement on your vehicle does not normally require the special socket for O2 sendors.
The number one upstream sensor is located between the motor and firewall, the number two upstream sensor is located between the motor and radiator, and the downsteam is located under the car. Replacing number one upstream and the downsteam sensor both require jacking up the vehicle to work from underneath. Replacement of the number 2 upstream sendor can be done with just the hood up.
First, unplug the sensor from the wiring harness.
Second, remove with a 22mm boxed or open end wrench.
Third, compare your purchase with the sensor that has been removed.
Replacement is the opposite of removal, install with 22 mm wrench and plug in.
You may have to disconnect the battery to clear the check engine dash lite and code.
on you exg. there is 2 02 sensors. upstream and downstream up is before the cat, downstrem is after. most 02 sensors are a bolt desing and hold themselfs in, so under the truck find the bolt with a black wire looking thing after the cat bolted into the exg., disconect it from the plug an play point (sohuld be somewhere under the truck, not to hard to find) and unbolt it. then reverse steps to install.
There are 2 bank of o2 sensors on Toyota Avalons. Bank1 sensors are on the exhaust piping. Sensor 1 is BEFORE the catalytic converter and is the one you need to look at. Although it is possible it just needs a good cleaning, it is easy to replace. A 22mm metric wrench and some anti-seize will do it.
Right after the catalytic converter, only about a foot away from the upstream O2 sensor. If you must replace either O2 sensor, I strongly recommend using only genuine Denso sensors, you can get a compatible (solder in) on for as little as $35 (see Amazon for example). Other brands do not meet Toyota's extra sensitive requirements. For the downsteam O2 sensor, you can also install a spark non-fouler ahead of the O2 sensor to avoid "catalytic convert efficiency below threshold" error code that is quite common on this car when it has high miles.
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