Any thoughts on oxygen sensors and a "PO420" code?

[MilesRacker]

OK,

I have a CA emissions 99 SR5 5spd 4wd, and I'm getting a CEL throwing a "PO420" code, defined as "catalyst system efficiency below threshold, (bank 1)." I know this code can mean a lot of different things. I'd like to start by testing my o2 sensors, does anyone have experience doing this or know of a good writeup? Also, do any CA T4R owners have opinions on brands or sellers of o2 sensors?

Thanks in advance, fellas.

 

[Cruiser64]

My 02 sport had the same code just the other day. If you search this fourm most have said that it is the o2 sensor on the back side of the cat, but more testing would be nessary to be sure. I just cleared mine and will see if it comes on again. This site is great for finding how to fix your problems. THANKS:awais: :awais:
I know this doesn't answer any of your questions but wait help is on the way.

 

[TheDurk]

If you are only getting P0420 I am not sure that there is any simple additional testing a car owner can do--there are ways to test heater resistance but your ECU is saying there is no problem there by not throwing other codes. The first thing to check is to see if there are any exhaust leaks before the cat. I would also suggest you reset the ECU and see how long it (the P0420 code) takes to come back--this will give an idea of the severity of the problem.

If no exhaust leaks and you have 70000+ on your sensors, then consider replacing those. P0420 results from a comparison of front and back readings, either can be a problem. Front sensors tend to go first and are also important feedback for the ECU in setting fuel trim. The rear sensor merely participates in throwing the p0420 code. This is the best place to get sensors:

URDUSA LINK

It is critical to get the correct ones for year, transmission, front/rear, and Cali. or 49 state--these guys have it down and sell OEM Denso sensors for a good price.


A look at the voltage curves from the sensors can help here if you have a way to look at them. I use a $100 dollar gadget called a BR-3 that connects between the car and my laptop. There are write-ups on how to evaluate the waveforms. In any case, most would replace both sensors before replacing the cats, which can cost $1800 at the dealer. Cheaper aftermarket alternatives are reported to keep throwing codes on these Cali. setups. Search P0420 here and on YotaTech for a wealth of info.

I have been getting p0420 about every three to five months for almost two years. I just reset and move on--I think the trip algorithm is a little funky as cats get older. The waveforms indicate my cat is still working when I look at them. I have replaced front sensor at 80K, and I'll do the rear if codes come more frequently. THEN I guess I'll end up doing the cats. There is a way a specialized shop can test whether front or back is problem but dealers don't do this--they just replace both. (I have a '99 Cali. 4Runner with 112k).

I am no expert, this just what I've learned on this and other boards since I started getting the code.

 

[scotts97T4Rltd]

If you are only getting P0420 I am not sure that there is any simple additional testing a car owner can do--there are ways to test heater resistance but your ECU is saying there is no problem there by not throwing other codes. The first thing to check is to see if there are any exhaust leaks before the cat. I would also suggest you reset the ECU and see how long it (the P0420 code) takes to come back--this will give an idea of the severity of the problem.

If no exhaust leaks and you have 70000+ on your sensors, then consider replacing those. P0420 results from a comparison of front and back readings, either can be a problem. Front sensors tend to go first and are also important feedback for the ECU in setting fuel trim. The rear sensor merely participates in throwing the p0420 code. This is the best place to get sensors:

URDUSA LINK

It is critical to get the correct ones for year, transmission, front/rear, and Cali. or 49 state--these guys have it down.


A look at the voltage curves from the sensors can help here if you have a way to look at them. I use a $100 dollar gadget called a BR-3 that connects between the car and my laptop. There are write-ups on how to evaluate the waveforms. In any case, most would replace both sensors before replacing the cats, which can cost $1800 at the dealer. Cheaper aftermarket alternatives are reported to keep throwing codes on these Cali. setups. Search P0420 here and on YotaTech for a wealth of info.

I have been getting p0420 about every three to five months for almost two years. I just reset and move on--I think the trip algorithm is a little funky as cats get older. The waveforms indicate my cat is still working when I look at them. I have replaced front sensor at 80K, and I'll do the rear if codes come more frequently. THEN I guess I'll end up doing the cats. There is a way a specialized shop can test whether front or back is problem but dealers don't do this--they just replace both. (I have a '99 Cali. 4Runner with 112k).

I am no expert, this just what I've learned on this and other boards since I started getting the code.

great post man! this is the kind of information that should be on here. im sure a lot of people will find this info helpful.

 

[TheDurk]

great post man! this is the kind of information that should be on here. im sure a lot of people will find this info helpful.

Thanks--it's cold outside and I had the time.

OH--MilesRacker--it's P0420, not PO420--that can screw up a search bigtime! On Google, you'll just get mailboxes :

 

[MilesRacker]

Durk,

Thanks for the info...very concise and comprehensive. I've reset the CEL several times and it usually comes back on, throwing the same code, within a tank of gas. I've noticed a 10-15% drop in mpg, so I'm hoping this gets me back to normal.

I'll get under it tomorrow and make sure there are no leaks.

Great post rating by Durk seconded. And thanks for setting me straight on the PO/P0.

 

[TheDurk]

Durk,

Thanks for the info...very concise and comprehensive. I've reset the CEL several times and it usually comes back on, throwing the same code, within a tank of gas. I've noticed a 10-15% drop in mpg, so I'm hoping this gets me back to normal.

I'll get under it tomorrow and make sure there are no leaks.

Great post rating by Durk seconded. And thanks for setting me straight on the PO/P0.

Aha! Given those two facts, I think you should look at replacing your front A/F sensor--read this:


LINKY

This write-up is based on non-Cali. truck so some of the code numbers are slightly different but everything else applies. Your truck and mine (identical) use a wideband Air/Fuel sensor in the front only with somewhat different characteristics but its function in the system is the same. The rear sensor (which is normal O2) has zero effect on mpg. The bad news is a messed up A/F can possibly do permanent damage to the cat by allowing excess unburnt fuel into it, and that might be what you are seeing--hopefully it is just the sensor and not the cat in your case.

How many miles on yours? If you decide to do it your-self, I can give you some tips; our truck is a little different than most of the write-ups. It's pretty easy to do.

 

[MilesRacker]

OK,

I'm ordering the o2 sensor, (front), tomorrow and I'll put this in upon receipt. I'll take any tips ya got since I'm all about asking those who have been there before. I haven't replaced an o2 sensor for 10 years or more, and that was on a chevy pos.

Yeah, hopefully this can be solved with a $79 part as opposed to a cat costing 10 to 20 times that...(yikes). I'm around 180k on the second front sensor, (according to previous owner's maintenance history).

 

[TheDurk]

OK,

I'm ordering the o2 sensor, (front), tomorrow and I'll put this in upon receipt. I'll take any tips ya got since I'm all about asking those who have been there before. I haven't replaced an o2 sensor for 10 years or more, and that was on a chevy pos.

Yeah, hopefully this can be solved with a $79 part as opposed to a cat costing 10 to 20 times that...(yikes). I'm around 180k on the second front sensor, (according to previous owner's maintenance history).

Uh, I don't think you can find the correct sensor for $79 (a Denso 234-9003). URD has it for 167.99. Please share what and where you are buying?

Installation tips:

1) if this Runner is running in Boston, assume the stainless nuts are toast and pick up 2 new ones from the dealer. San Diego, maybe not.
2) most write-ups say unplug the connector then remove the sensor. That won't work on our truck because the connector is on top of the exhaust. You need to unbolt the sensor first, then pass it back over the exhaust pipe. Then the wire will hang down so that you can unplug the sensor. To install, plug in, pass back over pipe, and bolt in (or nut in, actually) the sensor. Good luck!

 

[webster26]

Reviving an old thread here, but it has relevant info. First post here too.

I have a 99 sr5 CA emission, auto 4wd with 138k. I was getting codes before indicating a bad front A/F sensor (dont remember codes), and I tested the sensor and the resistances were not there indicating a bad sensor so it got replaced. Now a P0420 is coming on. I have reset it a couple times and it comes back on within a day or 2.

Here is my issue though, I tested the rear o2 sensor and it is coming in within the resistance spec that the manual gives for this sensor, leading me to think it is good. However, when replacing the front sensor the nuts were pretty worn down, and these back ones are almost non existent, leading me to believe this is an original sensor and maybe bad. So is the resistance test for these a good indication of it functioning properly? Or should I just replace it and see if it fixes the problem. I guess I want to save $50 on a new sensor if it is good, especially if I have a bad cat and will need to shell out money to fix that.

Are worn down nuts on exhaust components an indication of an issue? I do not believe I have an exhaust leak (I do not hear or see anything), but I am not 100% sure how to test for this.

Sorry for the length, but thanks in advance for the help!

 

[TheDurk]

Reviving an old thread here, but it has relevant info. First post here too.

I have a 99 sr5 CA emission, auto 4wd with 138k. I was getting codes before indicating a bad front A/F sensor (dont remember codes), and I tested the sensor and the resistances were not there indicating a bad sensor so it got replaced. Now a P0420 is coming on. I have reset it a couple times and it comes back on within a day or 2.

Here is my issue though, I tested the rear o2 sensor and it is coming in within the resistance spec that the manual gives for this sensor, leading me to think it is good. However, when replacing the front sensor the nuts were pretty worn down, and these back ones are almost non existent, leading me to believe this is an original sensor and maybe bad. So is the resistance test for these a good indication of it functioning properly? Or should I just replace it and see if it fixes the problem. I guess I want to save $50 on a new sensor if it is good, especially if I have a bad cat and will need to shell out money to fix that.

Are worn down nuts on exhaust components an indication of an issue? I do not believe I have an exhaust leak (I do not hear or see anything), but I am not 100% sure how to test for this.

Sorry for the length, but thanks in advance for the help!

The resistance check is relatively useless for a P0420. The ECM runs its own resistance check and if it is out of Spec it sets different codes (P1135 and others). Then you want to do the resistance check to see if it is the sensor itself or the associated wiring.

If the sensor heater is functioning, the resistance check will likely be good. It has no bearing on whether the sensor function itself is degraded, as will eventually happen with enough miles if the heater does not burn out first. This is the case where the rear sensor leads to a P0420 but no heater codes. So the resistance check is not helpful on a solo P0420. The resistance check will tell you if it is bad if it fails, but the sensor can still be bad if it passes.

Worn down nuts are perfectly normal if you see any salt. Checking for an exhaust leak is simple. Start the engine and climb underneath. Take the thinnest piece of paper you can find and pass it along the pipe. A stove mitt is recommended as leaking exhaust gas can burn you. Look for any displacement of the paper as you move it along the pipe, paying special attention to flange joints and sensors--but a hole can be anywhere. Go all the way from the rear sensor forward to the manifold.

 

[webster26]

Thanks for the info, very helpful. Relieved to know that replacing the sensor still has a good chance of fixing this CEL.

Truck has been in CO for most of its life, so lots of salt and mag chloride on the roads that could be responsible for the worn nuts. I thought the front ones were bad, but the back ones need a magnifying glass just to be seen. Hopefully that means there wont be much preventing them from being removed.

 

[TheDurk]

Thanks for the info, very helpful. Relieved to know that replacing the sensor still has a good chance of fixing this CEL.

Truck has been in CO for most of its life, so lots of salt and mag chloride on the roads that could be responsible for the worn nuts. I thought the front ones were bad, but the back ones need a magnifying glass just to be seen. Hopefully that means there wont be much preventing them from being removed.

The nuts are much softer than the studs, so you can crush them with Vise-Grips and then wire brush the leftovers off the studs. They are M8-1.25 stainless or 90179-08059 at the stealer.