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Help! How did I fix a P0420 by cleaning the MAF?

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6 years 6 months ago #12542 by Tyler
For the record, I've heard of this happening on other Toyota models before, but it's still weird. :blink: :silly:

This '01 Rav4 comes in complaining of the CEL, no other issues. P0420 stored, all monitors completed. FYI, this is a 2.0L I4 engine with two converters, two upstream A/F sensors and two downstream O2's:



Cool setup - buy one catalyst, you get the other included! :lol: I take a glance at the Mode $06 for the catalyst monitor:



No relevant exhaust leaks, sensors working... looking like a bad cat! I'm test driving the car when I notice it has a distinct lack of power. The customer wasn't complaining of it, but it's there. I can post the data if anyone is interested, but it led me directly to a dirty MAF. Low load, high trims during cruise, lean A/F sensors at WOT.

The customer isn't into buying a cat, but they're cool with cleaning the MAF. Understandable. So, I clean the MAF, clear the codes and recheck the data afterwards. Trims are pretty, power is back, everything is shiny.

The next week, it's back for brake work. I fully expect the CEL to be back on, but it's not. :huh: No codes, cat monitor completed...



Now I'm confused. :angry: I totally understand that catalyst performance is based largely on the feed gasses. But, if the MAF was causing the catalyst to fail, then it should have caused BOTH banks to fail, right?

The only explanation I have is that the bank one catalyst genuinely ISN'T performing as well as the bank two catalyst, but they're both above the fail threshold with correct fuel trims. The dirty MAF just happened to expose the performance difference between them. OR, the Mode $06 data isn't a good representation of catalyst performance, and the bank two catalyst was actually close to failing the monitor with the dirty MAF.

I'd love to hear your thoughts on this one! :cheer:
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6 years 6 months ago - 6 years 6 months ago #12547 by Chad
That's an impressive looking cat!

I don't get a whole lot of Toyota's and am not very familiar with the wide-band AF sensors, so this my not apply.

I judge catalyst efficiency by graphing Up and Down stream O2 sensors. Upstream should (mostly)switch rich/lean, while the downstream should (mostly) maintain a steady rich. Any excessive activity in the downstream makes me concerned for the cat. Symmetry between the Up and Down streams is a sure sign of low catalyst efficiency.

The second half of this video emphasizes the oxygen storage test.

As for how a dirty MAF could cause P0420, I would speculate a momentary fixed condition (be it rich or lean) produced symmetry? Why no RICH/LEAN code?...I don't know. Why only one bank?... I don't know.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

I have learned more by being wrong, than I have by being right. :-)
Last edit: 6 years 6 months ago by Chad.

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6 years 6 months ago #12560 by Tyler

pole71 wrote: I judge catalyst efficiency by graphing Up and Down stream O2 sensors. Upstream should (mostly)switch rich/lean, while the downstream should (mostly) maintain a steady rich. Any excessive activity in the downstream makes me concerned for the cat. Symmetry between the Up and Down streams is a sure sign of low catalyst efficiency.


Agreed, I look for the same. Unfortunately, the data rate is so painfully slow on this year of Toyota that it's very tough to get an accurate picture of the rear switch rate. :-( This is what I saw on the initial test drive:



Definitely a switching issue on bank one, especially compared to bank two. I didn't have as much time to play with the data on the second visit, but I did grab this Global data:



Still a switching issue. :unsure: The more I look at this, the more I have to think that the bank one cat is still on it's way out...

I remember that video! :lol: I thought of the oxygen storage test, and performed one on this vehicle. I used the bidirectional A/F Rich Lean test, which jams the fuel trims 25% positive and 12% negative on command. There's a clear storage difference between the two. Didn't have a chance to repeat the test afterwards. :-(



As for how a dirty MAF could cause P0420, I would speculate a momentary fixed condition (be it rich or lean) produced symmetry? Why no RICH/LEAN code?...I don't know. Why only one bank?... I don't know.


I had the same questions. :silly: Perhaps this has more to do with weak code set criteria than anything else?
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