2001 tacoma 3.4l o2 sensor - which one?

My check engine light recently came on and it threw out a p0420 code. It's been about 75000 since the last o2 sensor was replaced so I'm thinking that is the culprit. I'm having trouble figuring out which sensor I need to order though. I want to replace the upstream sensor, the one before the first cat on the exhaust.

Which sensor do I need for replacement?

$61 Denso 234-4189 Oxygen Sensor

Denso 234-4189? o2 sensor

or

$145 Denso 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

Denso 234-9001? a/f sensor

I think i just need 234-4189 o2 sensor correct? I'm just getting thrown off by the 'downstream' description since it's upstream from the cat.

Is it recommended to replace the downstream sensor at this time as well? Are they the same sensor?

Thanks for any help or advice,
Jay

 

I just replaced both of mine. I bought them on Amazon. First one was #

$170 Denso 234-9003 Air Fuel Ratio Sensor

Denso 234-9003 Air Fuel Ratio Sensor

And the other was

$61 Denso 234-4189 Oxygen Sensor

Denso 234-4189 Oxygen Sensor

Not sure which one was which but if you read the reviews of each you will see where the guys will say whether it's the upstream or the downstream one.

 

Thanks Joe.

From what I'm reading i think I need the a/f sensor for upstream and the o2 sensor for downstream?

can anyone confirm this for me?

Thanks again,
Jay

 

upstream is in front of your Cat or Cats

Down Stream is the one behind your Cat or Cats

It is the down stream sensor that monitors the difference between the up stream and down stream sensor detecting the Cat efficiency when there is not enough difference you cause the PO420 code

 

Got it.

Specifically I'm wondering which parts I need to order. It appears as if it's the a/f sensor is for the upstream : Denso #

$145 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

234-9001 : and the o2 sensor : Denso #

$61 234-4189 Oxygen Sensor

234-4189 : for the downstream. Is this correct?

Is it recommended to do both at once or start with just the a/f sensor?

 

First you do know that code can be anything in that Circuit or even a exhaust leak between the sensors even the cats could have failed .

The rear down stream sensor is Denso #

$61 234-4189 Oxygen Sensor

234-4189 which should be the first one you change and hope you get lucky!!

The up stream sensor has 2 different numbers depending if you have a auto or manual transmission

The manual is

$170 234-9003 Air Fuel Ratio Sensor

234-9003

The Auto is

$145 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

234-9001

These are all just plug and play remove your old ones Which can be a adventure in itself install the new ones.

Good luck !!

 

I am aware that it could be multiple causes through the system, but thought the most likely culprit would be the sensors since they haven't been replaced in 75000. i thought i would start there as well as cleaning the MAF.

Thanks for the help, I appreciate it!

 

It needs a converter.. if it had an issue with an 02 sensor it would set an 02 sensor code.

 

Wouldn't the upstream sensor actually be the one to replace first? The code is for Catalyst system efficiency below threshold (bank 1)

So shouldn't I replace the bank 1 sensor (a/f sensor) first?

The code doesn't mean it needs a converter necessarily, although it may

 

Think about this How could the Front upstream sensor do more then measure the exhaust before the Cats

It is the rear or Down Stream sensor that reads the difference and shows how the Cats are working.

Then it is your vehicle change them both

 

I bought both and only have changed the rear, downstream O2 sensor. I did the driving to get the computer reset and am happy to report so far no codes. I was getting Catalyst system efficiency below threshold (bank 1) code. The rig is running smoother and the smell of unburnt fuel is gone. I have put about 400 miles on since the change. Mileage is good and power is good. Good deal from Amazon!

 

Thanks toyrunner!

I just ordered the upstream sensor thinking since it was a bank 1 error that's what I should replace first. I've also noticed a dramatic drop in MPG since the light came on leading me to believe that it's the a/f sensor not the o2 sensor.

I also thought that since the o2 sensor is reading that the catalyst system efficiency is below threshold that it must be working in order to read that. Maybe I should just go ahead and replace that one as well though.

 

My rig has 330k on it so I just got both. I have had it for a year now and this is a good reference point for me. At the cost from Amazon, can't beat it

 

Yeah I've got 273k on mine so i figured doing both would be a good idea as well.

Thanks again for the help

 

It may indeed.. The code certainly doesn't say it needs 02 sensors

 

Have your cats ever been replaced? If so, with what brand/models? If your sensors are testing properly (fuel trim, voltage), and no exhaust leaks can be found between the front A/F sensor and the rear O2 sensor, it could very well mean that the cats are actually worn out and below the factory efficiency threshold. If that is the case, changing out properly functioning sensors for new ones doesn't solve anything, as the cats are the parts needing to be replaced.

 

That's a good question, I'm not sure if the cats have ever been replaced or not. I haven't tested the sensors for fuel trim or voltage, I'm not sure how to do that. Would I need a scanner tool?

I know that just replacing the sensors wont necessarily fix my issue and that it could be the cats or something else, but since toyota recommends replacing them every 75000 and it's been 73000 since they were replaced I thought it would be a good idea either way.

 

I just changed both on my 2002 automatic that threw the same code. Autozone said it was cat but I took it to my mechanic who did a temp reading and cat seemed to be within limits and no exhaust leaks either. The O2 sensor he said wasn't testing out perfectly so I decided to change it. I did both the rear o2 sensor (4189) and front "air fuel sensor" (9001) at the same time, not a hard job and Amazon had a good deal on the denso brand. So far so good and runs great. Both of those part numbers you have are correct.

 

Certainly no issue in following recommended replacement intervals. But at 273,000 miles, if those cats are original I can't imagine they're in good shape.

An OBD2 scanner would be required. I just use a Bluetooth adapter and the Torque Pro app, or you can take it to a reputable shop for some investigation. But looking for air leaks and testing the sensor outputs give you a better idea of where the problem is originating. Could it be the cats? Sure, but there are a lot of pieces and functions up-stream that can cause the problem as well.
Here's some reading that will better explain the aspects in play. Remember: knowledge is power!
http://www.autoserviceprofessional.com/article/92421/understanding-and-diagnosing-toyota-p0420-dtc
http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/

 

Right on thanks guys!!

Yeah after i left oreilly and the guy told me i needed new cats i searched and the first thing that came up was an article saying dont buy new cats! It could be the sensors, fuel filter, maf or even the engine coolant temp sensor...as well as it could be the cats. Im thinking that in general cats dont just go out (but i know they can) so im thinking the sensors are more likely to do that. Coupled with my dramatic drop in mpg it leaves me thinking its the a/f sensor.

Now the problem is Amazon doesnt have the part. Im wondering if i should wait to get the good price or if im gonna potentially burn up my cats driving it around in this condition? What do you guys think? Spend the money and fix sooner or wait for the better price and wait potentially 2-4 weeks?

Thanks again for all the advice!