Bad O2 Sensor or Bad Cat? How to Differentiate? Code 0420

[bicycleagent003]

Hey Y'alls,

I have a rig throwing code 0420 (1996 FZJ80) and according to the FSM that means the cat is not working properly. So....how do I know it's not one or two bad oxygen sensors? My mechanic (LC specialist) tells me we should replace the cat. But that's $350 (he says) to install the Magnaflow cat, which by all accounts is not as high quality as the OEM cat, and I still don't understand how you can know for sure it's the cat. Also, there are two cats. How is a person to know which is bad based on the code?

I did test the O2 sensors per the FSM and each tested fine, but that test is not visual only electronic and it seems the sensor could pass the test and still be faulty. Is that right?

Thanks for any help you can offer,
Russell

 

[Mrtwooooo]

I have replaced my front O2 sensor and not the rear one ( can't get the rusty bolts off). I also have some aftermarket cat on there from the previous owner. Just curious when your CE light comes on. Because I've gotten the code numerous times and it always happens when I'm going about 40-45mph with very little throttle and light engine load. At my last job I could just about predict when the CE light would come on. It always happened on the same stretch of road.

Right now when I get the code I just immediately clear it with the scangauge.

 

[tirewater]

I think your question is "how can I not pay $350 to fix my problem"

The FSM says you have a bad cat. Your mechanic says you should replace the cat. The FSM says your O2 sensors are fine.

Seems pretty clear that you need to replace at least one of the catalytic converters.

Did your mechanic test the NOX emissions of your vehicle?

The second O2 sensor determines if the catalytic converters are functioning properly. You can certainly replace that as well to verify the catalytic function. It only hurts your pocket book.

 

[LFD2037]

U can get a single universal Magnaflow Heavy Metal cat for $100 that'll replace the 2 you have now and still be code free. Google:
FZJ80 Magnaflow Heavy Metal Universal Cat.
Look for a thread where LandcruiserPhil and I are discussing it. He's the one that informed me about it.

 

[bicycleagent003]

My mechanic says I should not replace the cat, it isn't necessary yet. I appreciate when mechanics turn down money.
The mechanic says he ran the truck on his Scan Tool while driving it and the NOX is as it should be, and all is well. I guess that means the 02 sensors are not accurate.
Interesting findings, IMO. I'm leaning toward replacing the sensors. $100 gets you two Denso sensors on Amazon. What do you guys think?

 

[LFD2037]

Yup, order the Denso's from Amazon or Rock Auto. I just did!

 

[waterweber]

Be careful with the Non-OEM sensors. I had terrible luck with the ones I bought. They just didn't work worth a crap. Tons of posts on here. Pay the money and get the OEMs. This is coming from a guy who buys the non-toyota parts whenever possible.

 

[waterweber]

I think your question is "how can I not pay $350 to fix my problem"

The FSM says you have a bad cat. Your mechanic says you should replace the cat. The FSM says your O2 sensors are fine.

Seems pretty clear that you need to replace at least one of the catalytic converters.

Did your mechanic test the NOX emissions of your vehicle?

The second O2 sensor determines if the catalytic converters are functioning properly. You can certainly replace that as well to verify the catalytic function. It only hurts your pocket book.

Posts like this are absolutely worthless.

 

[ChiTown]

There is a way to check but you would need the ability to log the data from both O2 sensors to compare pre and post cat readings.

I've seen O2 sensors test correctly but still be slow causing this code.

In cars where that type of logging is not available I usually run a bottle of CataClean through it and reset the ecu (cheapest fix), if the code comes back then I'll replace the O2 sensors and reset the ecu (2nd cheapest fix), and if both those fail then I'll replace the catalytic converter.

As far as universal Magnaflow vs the direct fit, use the direct fit whenever possible. By the time you buy the universal one and pay the extra labor to have it custom welded in opposed to the 4 bolts of the stock one the cost isn't far apart not to mention future serviceability.

 

[LFD2037]

Be careful with the Non-OEM sensors. I had terrible luck with the ones I bought. They just didn't work worth a crap. Tons of posts on here. Pay the money and get the OEMs. This is coming from a guy who buys the non-toyota parts whenever possible.

I bet $100 you bought Bosch and not Denso. Denso makes the OEM ones. Denso's are 100% good to go and are OEM except won't come in a Toyota box if you buy from Amazon or Rockauto. But is that fancy red box worth an extra $75 to you? It sure isn't to me!!

 

[Hornd]

"Buy OEM it's the best" How often do we hear that!
The aftermarket stuff is from the company that sold it to Toyota, but that's junk since it didn't come from the dealer. Go to bed.

 

[CJF]

ANGRY thread.

From numerous posts on here, it's most likely just a bad rear sensor. That sensor's main function is to monitor the cat. efficiency. Problem is, the sensor itself eventually fails and throws a "bad cat." code even when the cat's are fine.

Look at it this way: O2 sensors are wear items, so if you did replace the cat's you'd also want to replace the sensor that monitors them anyway. So therefore, replace the sensor first and see if that's all you need.

 

[waterweber]

I bet $100 you bought Bosch and not Denso. Denso makes the OEM ones. Denso's are 100% good to go and are OEM except won't come in a Toyota box if you buy from Amazon or Rockauto. But is that fancy red box worth an extra $75 to you? It sure isn't to me!!

You are correct, i did get the Bosch ones. If the Denso ones are the same as OEM definitely get them. Sorry for the confusion.

 

[Parsnip]

<snip>...not the rear one ( can't get the rusty bolts off)...<snip>

Wire brush off the flakey rust. Naval Jelly to remove the rust. It may take two applications. Do it when the exhaust is cold. Put something under it to catch the drip.

Doorman makes replacement studs. They are cheap and sold at every parts store chain.

 

[tirewater]

Posts like this are absolutely worthless.

Eh, whatever. The point was he needed to do more testing. The initial post said his LC specialist mechanic said he needed to replace the cat. The OP follow up said the mechanic tested NOX and said cat was fine. Perhaps I could've been less condescending.

Here's the information on O2 usage in MY1996-2003 vehicles (Similar to what ChiTown posted):

[photo="medium"]26119[/photo] [photo="medium"]26120[/photo] [photo="medium"]26121[/photo]

 

[waterweber]

Eh, whatever. The point was he needed to do more testing. The initial post said his LC specialist mechanic said he needed to replace the cat. The OP follow up said the mechanic tested NOX and said cat was fine. Perhaps I could've been less condescending.

Here's the information on O2 usage in MY1996-2003 vehicles (Similar to what ChiTown posted):

[photo="medium"]26119[/photo] [photo="medium"]26120[/photo] [photo="medium"]26121[/photo]

Now that's a useful post!

 

[wrongway around]

do what every other redneck would do when confronted with a code 420 , find the source.