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Or it means that when at low RPM there is enough of a leak that it is not sending it downstream to be picked up by the downstream sensor.
I hope it works out for you.(y)
After fixing the exhaust leaks the fuel trim for Bank 1 has dropped from 10 percent to 4 percent which is good I think. There is an improvement in the O2 sensor waveforms as well. For low RPMs O2 sensor 2 waveform is nearly constant while sensor 1 wave oscillates. When I accelerate the O2 sensor 2 starts oscillating with sensor 1 somewhat but they aren't in sync like they would be if cat was doing nothing. Given this I think its prudent to wait on replacing the bank 1 cat. Since the engine fuel trim is in normal range and there are no misfires the risk that it will breakdown suddenly is low I hope.
 

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After fixing the exhaust leaks the fuel trim for Bank 1 has dropped from 10 percent to 4 percent which is good I think. There is an improvement in the O2 sensor waveforms as well. For low RPMs O2 sensor 2 waveform is nearly constant while sensor 1 wave oscillates. When I accelerate the O2 sensor 2 starts oscillating with sensor 1 somewhat but they aren't in sync like they would be if cat was doing nothing. Given this I think its prudent to wait on replacing the bank 1 cat. Since the engine fuel trim is in normal range and there are no misfires the risk that it will breakdown suddenly is low I hope.
@DrFeelGood I found that the test for catalytic converter efficiency must be done at Idle after the catalytic converter has reached operating temperature. Since I am seeing no inversions on the 2nd 02 sensor of bank 2 under this test condition the Bank 2 pre-cat is good and there is no need to replace it.
 

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Rally Racer
2004 Mazda 6s Wagon ATX
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Discussion Starter · #43 ·
@DrFeelGood I found that the test for catalytic converter efficiency must be done at Idle after the catalytic converter has reached operating temperature. Since I am seeing no inversions on the 2nd 02 sensor of bank 2 under this test condition the Bank 2 pre-cat is good and there is no need to replace it.
Excellent. If that is without a non-fouler in place for sensor two it is then a watch and wait scenario. Maybe check back from time to time to see if it has changed. 🤷🏻‍♂️
 

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Excellent. If that is without a non-fouler in place for sensor two it is then a watch and wait scenario. Maybe check back from time to time to see if it has changed. 🤷🏻‍♂️
I agree. Yes, this is without a non-fouler. I have a non-fouler only on bank 2 O2 sensor 2 which always shows a flat line with a hollow pre-cat. I always have a bluetooth OBD II connector plugged in so its easy to see the diagnostics on the phone.
 

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Excellent. If that is without a non-fouler in place for sensor two it is then a watch and wait scenario. Maybe check back from time to time to see if it has changed. 🤷🏻‍♂️
Check engine light for P0421 went away after driving 30 miles with the leak fix. So ECU is happy with the improved waveforms :) Here is the leak fix I did along with tightening the flange bolts more. The white gasket sealer is mostly Koalin which is the same clay used porcelain objects. Since it's baked now it should last a long time I hope. I had to put a parabolic heater under it for 4 hours set it otherwise it was soft even the next day in cold and wet weather here. The setting time in the instructions is 2-4 hours.
 

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Check engine light for P0421 went away after driving 30 miles with the leak fix. So ECU is happy with the improved waveforms :) Here is the leak fix I did along with tightening the flange bolts more. The white gasket sealer is mostly Koalin which is the same clay used porcelain objects. Since it's baked now it should last a long time I hope. I had to put a parabolic heater under it for 4 hours set it otherwise it was soft even the next day in cold and wet weather here. The setting time in the instructions is 2-4 hours.
My wife's 2006 Mazda6 3.0L had a bad rear catalytic converter causing lack of power from the engine and P2229 code. Bolted on a new BRE cat. Several weeks and several hundred miles later, I still have engine power, but now i'm getting a check engine light for P0431. Others on this forum state to change out the downstream O2 sensor. Rather than me throw parts at this, can you tell me what the O2 sensors should read on a simple scan tool? I was thinking that I should have thrown some high temperature Permatex gasket maker on the flanges of the bolt-on cat converter seeing how leaks can be the cause.
 

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Rally Racer
2004 Mazda 6s Wagon ATX
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Discussion Starter · #47 ·
My wife's 2006 Mazda6 3.0L had a bad rear catalytic converter causing lack of power from the engine and P2229 code. Bolted on a new BRE cat. Several weeks and several hundred miles later, I still have engine power, but now i'm getting a check engine light for P0431. Others on this forum state to change out the downstream O2 sensor. Rather than me throw parts at this, can you tell me what the O2 sensors should read on a simple scan tool? I was thinking that I should have thrown some high temperature Permatex gasket maker on the flanges of the bolt-on cat converter seeing how leaks can be the cause.
As I am sure you have read by now, P0421/P0431 means catalyst below efficiency. Everything you have asked is here in this thread. A P0431 is the right hand side and a P0421 is the left side. They tens to both go when there is a misfire and if you have not fixed that yet there is no point in fixing the converters.
The 1st O2 sensor controls the fuel ratio and it is moving constantly between .0v-1.0v as it adjusts the fuel injectors. The second sensor is only there to monitor the catalytic converter and it wants to read a relatively constant and steady voltage. If the second (downstream) O2 sensor is measuring the same or similar movement as the first (upstream) O2 sensor then the computer says the converter is no longer catalyzing and must be replaced. As sensors get old they react slower so putting a new sensor in will allow it to read a bad exhaust easier.

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As I am sure you have read by now, P0421/P0431 means catalyst below efficiency. Everything you have asked is here in this thread. A P0431 is the right hand side and a P0421 is the left side. They tens to both go when there is a misfire and if you have not fixed that yet there is no point in fixing the converters.
The 1st O2 sensor controls the fuel ratio and it is moving constantly between .0v-1.0v as it adjusts the fuel injectors. The second sensor is only there to monitor the catalytic converter and it wants to read a relatively constant and steady voltage. If the second (downstream) O2 sensor is measuring the same or similar movement as the first (upstream) O2 sensor then the computer says the converter is no longer catalyzing and must be replaced. As sensors get old they react slower so putting a new sensor in will allow it to read a bad exhaust easier.

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Its very possible that the cat near the radiator is bad because it was on the cars original engine which was missing and replaced with a remanufactured engine back in 2016. 30,000 miles later, I get this. Guess its back to Saturday mornings under the hood. Thanks for the replay and info. If the cat is bad, I really hope that the bolts dont break off.
 
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