P0420 -- Catalytic efficiency below threshold (Bank 1)

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Hi, I thought I might post this in a new thread to make it more visible...

Basically I started off with a check engine light ('96 Camry LE 2.2L L4). Code pulled came out P0420. Now I know this is a little out there, but with a little research I read about someone who got this problem when after he hosed down his engine and got his spark plugs wet. I, too, had done the same thing about three weeks ago so I figured I might start from there. Changed the plugs -- which were in fact a little wet :oops: , pulled the EFI fuse to clear the code, and it hasn't come back on since Sunday morning and at least five drives.

But, I decided to go ahead and test the O2 sensors, anyway, (and most likely will look at the catalytic converter down the line). The results of the O2 voltage measurements are below:

The voltage at the main O2 sensor (white wire, Bank 1 Sensor 1) climbs steadily from 0 V to 0.9 volts when I first turn on the car (cold). Then when the engine is hot, it fluctuates from 0.2 to 0.9 V. According to the Hayne's manual, this is proper behavior for the sensor.

The sub sensor, on the other hand, only got up to .4 or .5 V and on the down turn will sometimes go all the way down to .04 V. Is this automatic cause for replacement, or could something else be wrong? Please help.

posted by  JaneiR36

It does help. The only thing is I'd like to know how exactly to get those results? I only got the fluctuations. Maybe do the readings after a longish drive (to ensure a hot engine)? And would I have to rev the engine while taking the measurements or anything? Please let me know, I think I'll have to redo the tests. Thanks again for your help.

posted by  JaneiR36

While this is not completely incorrect information, it is partially incorrect and therefore misleading.

The upstream HO2S should cycle between .1v and .9v, at least three times per second. Unless you have a fairly high dollar scan tool you won't be able to read it in real time and will have to be more concerned with watching for a steady variation. If the upstream sensor remains in the .4v to .5v area it is considered inefficient (lazy) and needs to be replaced.

Additionally P0420 does not indicate a bad cat, it indicates you need to perform further testing. It could also be caused by, and most often is caused by, a lazy downstream HO2S. This is simply a case of jumping to conclusions without complete diagnostics... And you know how much I like that.

posted by  vwhobo

A fully warmed engine is ideal but starting with a cold engine you can watch the both of the HO2S's warming up and going into closed loop. My recommendation is to do both, cold start and after a few miles of driving.

posted by  vwhobo

It's bright and sunny today so I finally got the test done. The main (upstream?) O2 sensor continues to give the same results while the sub sensor is still not making it to the expected limits on each side. It's no longer going as low as .05 V, but goes down to .093 V or so. The highest reading I saw on the upswing was .709 V, usually it would go up to anywhere from .4 to .6 V. This test was done after driving for about 10 minutes. Should I replace the sensor?

Also, the check engine light has not come on since I changed the spark plugs and pulled the EFI fuse. Does this say anything?

posted by  JaneiR36

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