'99 Camry DTC P0420

Is there any way to check the O2 sensors with a volt/ohm meter? The factory manual says to use an oscilloscope but I do not have access to one. I get a 0420 code (catalyst efficiency below threshold)But I would like to make sure it isn’t just a lazy sensor. It is my understanding that this car has 2 Cats, 1 at the exhaust maniflod and 1 in the pipe ahead of the muffler. How would I know which one has gone bad or do you just replace both (expensive). By the way the emissions sticker says this car approved for CAL & OTC OBD II certified.

The code you got refers to the first sensor in bank 1 which is closest to cylinder 1. Here is a link that may help you with that code.

Catalyst System Efficiency
Below Threshold
(Only for California Spec.)
Three?way catalystic converter
Open short in heated oxygen sensor (bank 1 sensor 2) circuit
Heated oxygen sensor (bank 1 sensor 2)
Open or short in A/F sensor circuit
A/F sensor
I got this from the Factory manual. I know where the 02 sens. is but don’t know if I can check it with a volt/ohm meter or need the Toyota scan tool that has an osilloscope type read out. And since the car has 2 cats which one or both need to be replaced?
Thanks for the quick reply.

Four Cylinder or Six? I’m pretty sure that for the four at least, there is only one catalytic converter and one upstream O2 sensor for non-California cars (I replaced my cat last Summer and spent no small amount of time staring at the exhaust set up while I cut/chiseled/broke rusted bolts off).

It’s possible to find the O2 signal wire in the harness, push a pin into it and get to the signal. I’ve done that. But the signal will probably be changing pretty quickly. You might be able to make some sense of it with an analog meter. A digital meter will almost certainly be hopeless. If you have a notebook computer, your best bet would probably be to invest in an adapter that will allow you to access the OBD interface through the connector under the dash. Mine came with software that allows me to monitor O2 sensors and I would hope that yours would also.

Sorry it is a 2.2L 4cyl. with about 61K on it. There are 2 cats. 1 at the exhaust manifold and 1 in the pipe before the muffler. there are also 2 O2 sens. 1 at the manifold and 1 after the rear cat. I am pretty sure it is a Cal. car.

Here is the emission sticker

Since the sensor is the first one in line the CAT that is refered to is also just after the same sensor.

Since there are no other codes then I kind of doubt there are problems like open heaters in the other sensors. I would first check for a intake manifold leak after the the MAF sensor or an exhaust leak before the O2 sensor as the cause for this trouble.

YES, this code CAN be thrown by a bad rear (behind the converter) sensor…As a matter of fact, it is this sensor that is throwing the code and causing the light to come on. Replace it before you replace the converter…These sensors have a life of around 100K miles…

No intake or exhaust leaks that I can detect. Where is the MAF I cant find one?

I replaced the rear O2 sensor, still getting the CEL code P0420. This car has 2 Cats. how can I determine which has started to go. 1 right after the exhaust manifold and 1 just before the muffler. It will cost about $600 to replace both just in parts. Could both Cats. have gone bad in just 61K miles?

On the 5S-FE engine the two oxygen sensors straddle the front catalytic converter. You will see sensor 1 poking through the heat shield covering the front cat when you open the hood, up high by the head. You can see sensor 2 down lower near the bottom at the level of the oil pan. It is before the flex section of the front pipe. Getting under the car will allow you to see it better.

The second catalytic converter follows the flex section of the front pipe and is not sensed by the ECM and so will not set the dreaded P0420 DTC. Thus, problems with the second catalytic converter will only show up on your emissions inspection readings as higher NOX and HC readings. Remember this vehicle is a Practically Zero Emissions Vehicle and if everything is working and warmed up, it can get the numbers well below the upper limit.

The reason the manual recommends using the oscilloscope is that you want to visualize both O2 sensor voltages at the same time. If sensor 2 switches coincidental with sensor 1, the catalytic converter is not binding oxygen i.e. the active matrix has lost its ability to remove oxygen from NOX and add it to unburnt hydrocarbons. If you use a high impedence 10Meg DVW you can see the voltage activity of the O2 sensors to verify their ability to respond to the presence or absence of oxygen. However, unless you have two DVMs connected to both sensors at the same time you are not going to be able to ascertain switching coincidence.

It is possible that a more capable Diagnostic Scanner tool which can read real time data can to used to display the ECM’s captured data from the sensors. You might be better off to let a emissions specialty shop diagnose the defective component. They will probably charge you a diagnostic fee if you want to replace the defective component yourself.

Please post back how you correct this problem. Good luck on this.

I went out to double check myself and found the rear sensor is indeed after both Cats. at the front of the resonator pipe part #48319 in the following diagram. So I guess I will have to at least pay someone to tell me which Cat. to replace if they can even tell.

Thanks you all for the replies. I think I have solved the P0420 code problem. The car was always driven real easy almost never exceeding 2500-2700 RPM. Every time I reset the CEL It would return within 2 days (same P0420 code). I reset the light, took the car out on a 2 hour 65MPH w/o overdrive (3400 +/- RPM) run. I haven’t got the code for almost 2 weeks now. I saw “run it a little hard” suggestion on another link.