O2 sensor removal

Yes, the wire enters the passenger cabin. You'll need to remove the "instrument panel finish panel lower center" which is the colored plastic trim piece above the carpet and below the center console. That is held on with four claws and two clips.

You may need to first remove the "instrument cluster finish panel center" which is below the stereo controls, and is held on with two screws and four clips.

Please say more about why you are replacing that sensor: odometer reading and DTC?

 

Hi Eric,

If you don't currently have a check engine light on, then there's no current DTC logged.

The O2 sensor located downstream of the catalytic converter only has the job of reporting how well the catalytic converter is doing. It has no other function.

If I was going to replace a sensor prior to a solid DTC being logged, it would be the air/fuel ratio sensor located upstream of the catalytic converter. That sensor is used to help the engine ECU determine the appropriate amount of fuel injection.

At least by replacing the A/F ratio sensor at 90K miles, you have some hope that your efforts might be rewarded with slightly improved mpg.

 

THAT is where the upstream O2 is for replacment its easy

 

tf4624, not me, should get credit for supplying the pictures...

 

Stan, I don't have the pics, but the sensor that needs to be swapped first is closest to the ICE in FRONT of the CAT. Can't get much simpler IMO. I am not sure if the code you have refers to that sensor, but FWIW, the front sensor is the important one. The rear sensor only tells you if the CAT is working. I would think it would be, otherwise you would probably have other codes showing up.

I could be wrong. Hopefully Patrick Wong will chime in again. I was lucky, never had to mess with those things on our Gen II.

Best of luck to you,
Ron (dorunron)

 

It is true that DTC P0138 points to the rear oxygen sensor which is downstream from the catalytic converter. The center finish panel below the radio has to be removed for access to the wiring harness connector that goes through the cabin floor, out to that sensor.

 

There you go Stan, I stand corrected. Patrick knows more about this than I do.

Ron (dorunron)

 

hey buddy I just had the same problem and it is common that it calls out the back but it is really the front that needs to be replaced.

 

Patrick, my 2004 Prius (original owner) now has 292,000 and I recently changed the spark plugs since I got check engine code: P0301 and P0420. P0301 is gone but P0420 continues after driving around 70 miles. The dealer recommended replacing the cat. converter for $2200 and $600 for the sensors. Having said that I would prefer to replace both front and rear Oxygen sensors first since it is less costly than replacing the cat. converter and see if the code P0420 comes back. I just got a quote from Midas and the upstream oxygen sensor is $322.50 and downstream $170.00 and cat. converter is $882.00 (far cheaper than what the dealer is quoted me). What is your recommendation with regards to taking process of elimination first as well as going with Midas instead of the dealer considering the age and mileage on my Prius. I intend to keep my current Prius for another 100,000 miles at the most and replace with 2017 Prius. Many thanks and I look forward to hearing your opinion and recommendations.

 

At your 292K mile odometer reading there is no reason not to replace the sensors. I would replace the front air/fuel ratio sensor first, and see what happens.

If you are going to buy online then buy from one of the Toyota dealers that sell at a discount over the web, for example:
Toyota Parts - Genuine Accessories | AutoNation Toyota Gulf Freeway Parts
or parts.com

I suggest you confirm with your dealer's service writer or service manager that they are willing to install parts which you provide. This is like bringing your raw steak to a restaurant and asking them to prepare it for you.

 

Patrick, many thanks for your quick response. I edited my previous message since I just got a quote from Midas. Your thoughts using Midas over the dealer is much appreciated.

 

If you're not in California, you can buy an aftermarket cat online for about$100. Then have a muffler shop install it

 

nope in NJ but many thanks for the tip.

 

Magnaflow 99205HM Universal Catalytic Converter.

I personally wouldn't want to throw money at a car with that many miles

 

Well, if the choice is between Midas vs. the dealer, it seems like the Midas choice might be reasonable.

JC offered a reasonable suggestion about having a universal catalytic converter installed. Don't expect it to have a very long service life, perhaps 50K miles if you are lucky.

I would start by replacing the air/fuel ratio sensor. If that didn't help then I would have a universal catalytic converter installed.

I would not be in a rush to replace the rear oxygen sensor because a new rear sensor is more likely to detect a problem with the failing cc - the indication of good cc health is that the rear sensor voltage signal is relatively stable while the front sensor shows widely varying voltage consistent with closed loop operation where the air/fuel ratio swings from rich to lean within control limits.

A new rear sensor, being more efficient, will be quicker to demonstrate voltage variations vs an old sensor.