ssgi4in12

Have been battling with this annoying catalyst code and I have been unsuccessful Diagnosing what the actual culprit would be. Since it was for bank one u had to check the cat on that back and it looked pretty okay. Now I am thinking my downstream 02 sensor could be responsible.

I was advice by some tech guy to idle the engine at 2500 rpm and what the voltage of the downstream 02 sensor. I noticed the sensor was reading out at almost a straight line on 0.1v. I had read in some other posts that the sensor should oscillate between 0.1v and 0.9v and not remain steady at 2500 rpm. Can someone help confirm this? Or am I missing something.

ssgi4in12

Edit :

​I made a screen shot of the torgue pro app while driving Driving at almost 70km /hr
Stopped at traffic

Lexmex

You should get oscillation. I have been down this road before and replaced the sensor (and I also use a an aftermarket cat from magnaflow).

ssgi4in12

Thanks lexmex. The graphing is for the 02 sensor bank one sensor two showing as 02 1 x 2v and the af sensor bank two sensor two sensor 1 showing as 02S5LV. The af sensor bank 1 is mirroring the same reading like the bank 1 af sensor. Am I supposed to get an oscillation on the 02 sensor graphing? I had to pull the 02 sensor to read on a multimeter and it showed continuity. Could the sensor be dead even when it reads out?

I brought down the bank 1 cat and did a physical examination, it sure looks in good condition. Could changing only the cat underneath the car solve the code?

ssgi4in12

I have been stuck trying to figure out what could be causing my P0420 Cel for 4 months now. And I wouldn't just want to throw money except I am confident it is the actual culprit.

​​​​​I fixed an exhaust leak just before the downstream 02 sensor but the code returned after 4 days. Brought down the bank 1 cat for physical examination, it looked quite in a good condition and it is not clogged. While fixing the leak after the downstream cat, I saw it was still intact as well. I have replaced the gaskets where the cats join. But still my P0420 keeps coming Back.

Now I want to figure out if it is the 02 sensor that is defective or replacing the downstream cat.

I will appreciate anyone that can help me with the solution.

Here are some pictures

​​Cel
Freeze frame
Mod 6 test results
02 sensor reading on idle
I quickly accelerated to 2000 rpm back and forth for 3 times
At 2000 rpm
At around 2500 rpm

fastnoypi

test the heater circuit of the suspected o2 sensor with a multimeter. If no resistance is found, it is confirmed bad.

It is seldom the fault of the cat.

ssgi4in12

I have had the 02 tested on multimeter. It shows continuity. It was reading around 0.16

fastnoypi

Try logging B1S1 and B1S2 voltage output at the same time. The engine has to be warmed up to be in closed loop. If the B1S2 mimics B1S1 it means your catalytic converter is either bad or clogged.
B1S2 should graph alot smoother than B1S1 if working normal...showing the catalytic converter is doing its job.

Since this is a 1st gen, you might have clogged the cat with oil residue due the the known oil consumption caused by the rear valve cover design. Assuming the B1S2 sensor isn't faulty and short of replacing the cat, you can take a long spirited highway drive to get the cat nice and hot to attempt to burn the oil residue out.

ssgi4in12

Thanks for your response. I have physically inspected the two cats on bank 1 and they look pretty good. I tried logging the output voltage of b1s1 and bank 1s 2. I am attaching the pictures. Do you have a picture of what the ideal output should be?

The car is idling here after about 15 minutes of city driving. What does this show?

fastnoypi

please confirm which sensor is which in your attached pics. Which is B1S1 ? Your logger identifies them in a non-intuitive way..but your graphs would lead me to believe the 2nd graph is S2

ssgi4in12

The first graph is the bank 1 sensor 2. The 02 sensor after the downstream cat and the second graph is the af sensor bank 1 sensor 1.

fastnoypi

Your b1s1 is malfunctioning if it is your 2nd graph. It should have alot more oscillating activity. Replace it with an OEM lexus o2 sensor or Denso equivalent, do not use Bosch.

ssgi4in12

Thanks for your assistance so far. The bank 1 sensor 1 and the bank 2 sensor i.e the two a/f sensors shows the same reading. Could the two of them be bad? Lexmex said the bank 1 sensor 2 should have a lot more oscillation. Now I am getting really confused. Pls do you have a picture of the oscillation of the two af sensors?

If the two af sensors is malfunctioning, would it not be throwing a cel code?

fastnoypi

B1S1 should always have more oscillation than B1S2...it is the o2 sensor immediately after the exhaust manifold to give feedback to the ecu for fuel adjustment. B1S2 is more for monitoring the cat efficiency.
Here is a youtube video that would explain the expected oscillations differences between the two type of o2 sensors.

ssgi4in12

​​​​​​
Many thanks again for the video. Actually I have watched lots of videos regarding Diagnosing an af sensor. The voltage of the bank 1 sensor 1 and bank 2 sensor 1 should be 3.30v on Toyota (lexus included) an it constantly remains at that. When it is it increases when it there is a lean condition and decreases when there is a rich condition. I have done those test to confirm that the af sensors are responding according. On the scan tool on that video the af sensor has the same range of 0 - 0.9v like the 02 sensor. If I have scanner with that range for the af sensor, i think it might be oscillating.

https://m.youtube.com/watch?v=XQYDwIWS6Ho