Downstream O2 sensor operation - P0420
 

Toyota Highlander 4cyl.

P0420 catalyst efficiency code. Scanner shows voltage swinging between one and eight tenths of a volt rather than a steady .45 volts. Classic symptoms of a done for cat.

My question is if there is a small exhaust leak a foot downstream of the rear O2 sensor (at most a pin hole) AND outside air was going upstream that foot to the sensor wouldn't it simply read high all the time? I am pretty sure this would be the case. This scenario assumes the cat is working properly.

Make sense?

I had a code like that about 18 months ago on my Chevy HHR with the 2.4L engine. I checked on the main HHR forum and several posts noted it was a failing cat. I also checked with a mechanic I trust who has worked on both or GM car and he said the same. Sooooooo I had to but a new cat but in CA you have to get it from the dealer $1200 but Rock Auto has it for $200 but won't sell it to me because of smog laws. I can't tell you how I did the replacement but I got the $200 one, same part # as the one the dealer wanted to sell me.

If it is a failing cat check your smog laws in the state you live as yours could be aa bad as ours?!

Not even close to the CA stringent rules in OH. I believe NY is pretty fussy though.

Thanks.

I know that if there is a leak before the downstream sensor it can sniff the outside oxygen leaking in and throw that code but I believe that it would signal "rich" all the time. The downstream sensor usually has no impact on fuel trim like the upstream ones. I may have a pin hole leak downstream of the rear sensor but I am asking if that outside air could reach the o2 sensor would there be voltage dithering or would it just peg to rich? Pretty sure it would peg high in the v range.

Fresh air through a pinhole won't suck back up the exhaust stream that far. It won't sit at a constant .45 either. It will be a slow gradual drift back and forth following fuel trims when working properly.

Agree but it's replicating what I have seen on old style front O2 sensors. An eight tenths of a volt swing. I would buy a .35 to .55 swing as ".45". This car is outside that range.

Thanks for the info. I don't think fresh air could fight it's way back up either.

The front O2 sensor is an air fuel sensor which is different than old style O2 sensors. It's in line with what I have read is normal. Just a wee bit over three volts.

I am just asking some questions before having customer drop the coin on a cat.

I went through this with my son's Tacoma PreRunner. Toyota has an issue with P0420 code and how the trim is very tight compared to any other car. I replaced both O2 sensors and the cat before the issue went away.

I have a graphing OBD reader so I was able to graph the voltage changes but still had to replace all the parts.

Good luck with fixing it.
Dave

That was my experience with an 03 Tundra.

Thank you.

I have seen it on RAV4's, Tacomas and now this Highlander.

I have a code for the rear sensor (replaced yesterday) but not the front. Time for a cat. Pretty high mileage. Over 175k.

2004. It's time.

Around here, that repair would have amounted to splicing in a straight section of pipe...

Depending on the county same here.

Would a bad sensor do that?

Flyin' Miata finally produced an after market downpipe for California. SS, lighter, flows better and half the price of the OEM part. Probably won't last as long as the stock one, but I can always buy two, since they are half as much.

I don't think so. Separate OBD codes should surface if sensor is out of range or heater is bad.

I am convinced that P0420 is bad cat or air leak in front of rear O2 sensor.

I did not take a scanner read of rear O2 sensor voltage before I replaced it. That's a bummer.

I went through this years ago on a Tacoma. Essentially, P0420 means the catalytic converter is operating at a lower efficiency than it is supposed to. This "efficiency" is typically measured by the difference in the upstream and downstream O2 sensor readouts. The thing of it is - a P0420 code doesn't necessarily mean you have a problem. Toyota set this up to meet CA emissions, so the required efficiency of the cats is much higher than what is required in the other 49 states. The vehicle will throw a P0420 code when the cats drop below that CA efficiency level, but they are typically just fine (legal) for everywhere else.

I replaced O2 sensors and mine never went away. The only way to really solve it is to replace the cats, but that is a nightmare as the Toyota cats are a fortune and I've read lots of stories of aftermarket cats still throwing P0420 codes. I bought a scan gauge and simply cleared the code when it would should up.

There are places online (he says in a hushed voice...;)) that sell a resistor or something to put in place of that downstream cat to prevent this P0420 problem as it is very prevalent on Toyotas. I would check to make sure you don't have some obvious problem that needs to be addressed (exhaust leak, etc.), but the "problem" you are having is very common on Toyotas.

I have heard that you can put a blank plug in the downstream sensor port, plug your oxygen sensor into the harness and set the sensor under the seat. Downstream sensor then sniffs clean air and thinks the cat is doing great. This would be circumventing your emission controls, and that would be wrong.

There's an app for that!! SmileWavy

https://www.urdusa.com/rear-sensor-simulator/

One spark plug non fouler, please! Works like a champ. 340K, no need for a new cat.

I need to dig into this. I read about it when digging into the below paragraph years ago.

The resistor trick is to solder a resistor in line on the signal wire from rear O2 to the ECU. Easy and it muffles the voltage swings. Couple thousand ohms resistor.

If one did this on their own car and got caught, maybe a minor fine.

If one did this on a car they received money for to fix, I see potential incarceration.

However. Good data.

Look away. :D

I have a Bosch Bluetooth code reader that will live stream data. As I understand it, your down stream should be fairly constant while the up stream sensor will fluctuate. If you have a code reader that will live stream you could see what the outputs are and tell which sensor is failing or whether it is your cat.

In post one I said I have a scanner that says it is swinging wildly.

This is in conjunction with identifying the rear O2 sensor was kaput and post replacement indicted big swings in rear O2 sensor swings.

It is a Snap-On MT 2500.

Have heard that method works too. Have the capacitor, resistors and potentiometer, but haven't tested yet. I can't take them back to Radio Shack now, so maybe one day I can report on my findings.