Generation 2 Prius o2 sensor vs air fuel sensor locations and how to install

Hi you all, OK, lets get down to business. In checking out other threads on this subject, I still see confusion about what even an o2 sensor is. The 2nd generation Prius has just a single o2 sensor and a single air/fuel sensor.

o2 SENSOR: This is the sensor below the cat converter, and hence "downstream". It sets behind the converter right below the passenger side compartment. If you have the correct tools, this one is fairly easy to replace. There is nothing hiding it as it sticks out of the right side of the pipe. You can use an 22mm open end wrench to remove it (i'd suggest you spray it with PB Blaster before doing so. The electric wire for this sensor goes through the front passenger side floor board. Pull the lower center consul piece away from the carpet on the passenger side and pull the carpet back far enough so you can see the o2 sensor wire coming through the floor board and follow it to the connector. Remove the plug from the connector, push the rubber seal down through the floor board and drop the plug down through. To install the new one, put some anti-seize onto the new sensor thread and screw the sensor into the exhaust pipe and tighten it securely. There is a spring type wire holder on the old sensor. Install that on the wire just behind the sensor and make sure it is holding the wire up and not down. Then feed the sensor wire up through the floor board and push the rubber seal up in place in the floor board hole. Plug in the sensor and replace the carpet over the wire and push back the lower dash center piece into place. Lower the car jack and you are done. At first, I would not reset the PO420 code. Drive the care around for a day or two and see if it resets on its own. Mine did. If it does not resent, either use your scanner (if you own one) to erase the code. You can also unplug the 12v battery for about five or ten minutes, which will also erase the code. But using the scanner is much easier, because to disconnect the battery, you have to take some of the rear hatch panels apart to do so, which means emptying your reach hatch compartment, pulling the floor board out along with the plastic tray underneath to do so. Now this is the easy sensor to deal with. And remember this is the DOWNSTREAM SENSOR AND IS THE ONLY O2 SENSOR IN YOUR PRIUS!!!

Now for the upstream air/fuel sensor: This is a much more difficult job depending on the year of your Prius. If you own the 2004-05 model, you can get to the AF sensor from below, but you still have to go down into the engine compartment to unplug that sensor (can be a pain in the A**!). To remove this sensor, you will definitely need the special 22mm socket you can easily get from ebay for about $6. Harbor freight sells them for $7.99 (offset oxygen sensor wrench 3/8".) It is a tight squeeze from under the car as the sensor sticks out to the left. But it can be done. It is located on the exhaust manifold in front of the cat converter. Hope you have small hands lol). Once you remove the sensor, you have to go into the engine compartment. To get at the sensor plug, you have to remove the Windshield cowl completely, including the windshield wipers, motor and the metal cowl plate underneath. Look down behind the engine center left and you can see the wire from the sensor. Trace it to the outlet connection and unplug (its not easy as its in a tight place.) To make things easier the next time I might have to do this job, I went to a pick-pull and cut the outlet plug out of an 07 Prius. Then I cut the plug off the old sensor and spliced the outlet plug I got from the pick pull and spliced it to the other end of the plug from the old sensor. Then I inserted the old plug into the outlet wire and was able to plug in the new sensor to the outlet plug from the spliced piece. I made sure to make the spliced piece long enough to reach the new sensor. Then zip-tied the spliced connection safely out of the way. This way, next time I need to replace the SF sensor, I don't have to fight my way through the engine compartment to unplug the sensor which was then easily within reach . It worked perfectly. Thais was in the 2004 I once owned.

Now, unfortunately for my 08 Prius, it is impossible to reach and get a socket wrench from below the car. So, I had to do all the things I had to when unplugging the sensor wire on the 04. Before unplugging the wire from the sensor, I took a long 3/8 extension, put a 3/8 universal joint socket to it and placed the other end of the universal joint into the 22MM special socket. I had to reach way down, while lying on the engine and hand-placed it into the sensor. I lucked out that it freed the sensor quickly. Then I unplugged the sensor and removed it. I took the new sensor, plugged it in and reached down to screw it in to the manifold and then tightened it in reverse. Probably took me about 2 hours to figure things out, and that not including the time to remove all of the windshield cowl wiper crap. Felt really good when i finished! Wasn't sure I was going to have success! The way the sensor is placed, I could hardly see anything I was doing! I did it all on feel.

Sorry, but no pics. But hopefully this helps someone!

Once again though, please remember that you have an upstream Air/Fuel sensor and a downstream o2 sensor. And that the PO420 code usually means a problem with your o2 sensor. When I first had this code show up, I replaced the air/fuel sensor and that didn't fix anything. So I replaced the o2 sensor and the code never came back! I basically wasted my good money on an expensive AF sensor that I didn't have to do!