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Discussion Starter · #1 ·
2006 Accord LX 4 cyl
132,000 miles

Got a P0420 CEL code.

And, of course, my registration is due in mid-March and this is the year I need to get a smog inspection (I live in California).

Money is super tight, so in order to save the $100+ on a diagnostic, I've been trying to figure out whether it's my catalytic converter or not. I got a cheap Bluetooth OBDII scanner and using that with Torque on my Android phone.

Anyway, when I check the upstream O2 sensor, there's no reading at all (I checked on my other car and the scanner sees both sensors, so I don't think it's the scanner). Downstream O2 sensor gives me the fluctuating pattern that the upstream is supposed to show. So I'm thinking I've got a bad upstream sensor, which would explain why the downstream sensor is giving the fluctuating reading.

Pulled the upstream sensor, heated it up and did a voltage test with multimeter. After a few minutes, voltage starts doing the .2-to-.8 dance as expected.

Next I called the Honda dealer to ask about replacing the catalytic converter, and during the conversation with a service technician, he basically guaranteed me that a P0420 is always the catalytic converter and that I shouldn't even bother bringing it in for a $99.95 diagnostic check. So the fact that he was willing to forgo the extra $100 tells me he's probably being honest.

So now I'm not sure which direction to go. Here are my real questions:

- Is the upstream sensor bad? If so, shouldn't I also be getting a code for that?
- I've read conflicting reports on aftermarket catalytic converters and I live in California so my options would be limited. Is it worth saving what appears to be $300-$400 with an aftermarket converter or will I just end up replacing it in two years and end up having paid the same as I would have with an OEM converter (Honda quoted me $790 for the part)?
 

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Here is what I would like to know, is the ECM operating in closed loop or open loop mode? The engine can operate without an operating O2 sensor but the ECM would be in open loop mode. Have the readiness tests completed? I would expect that you would have a different error if sensor 1 has failed something like P0130, P0131, or P0134. You should read these pages


http://www.easterncatalytic.com/education/tech-tips/pesky-p0420-codes/

http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/


You could just replace the Catalytic converter and that may fix it, but Catalytic converters do not just wear out, they fail from fuel/air related issues and if you do not fix what made the Catalytic converter to fail the first time it will happen again.
 

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Discussion Starter · #3 ·
Thanks for the reply greg1c. I was doing the test in an open loop, so I guess I'll try a closed loop before jumping to any conclusions.

Another thing is that the car just sounds ... bad. Almost like there's something rattling around under the power steering pump. My dad (who doubles as my mechanic consultant) said it sounds like it could be a bearing.

Any ideas if this could be related?
 

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The ECM will go into closed loop when everything has passed the readiness test and the car is at temp. As far as the noise, that could be a belt tensioner problem. Take the belt off and start the engine and see if the noise goes away or not. If not I would suspect an issue with the timing belt and/or tensioner issue.

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Any thoughts on the aftermarket catalytic converter?
I would use an aftermarket Catalytic converter, You can not use an aftermarket in New York and California unless it is CARB

OEM price is $453.40

On Rockauto you can get a Walker 16299 direct fit Catalytic converter for $134.79 (not carb compliant)


So I would use a an aftermarket for that price difference. You may not need a new Catalytic converter, your first have to fix why you are currently getting a P0420 code. If you need a new Catalytic converter and do not fix what caused it to go bad the new one will go bad as well.

In California and New York you can not use non-carb compliant replacement Catalytic converter, so if you are in California or New York you will most likely have to get a Honda OEM converter.
 

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Discussion Starter · #7 ·
For fun I ran another scan and now I no longer get the P0420, but I'm getting P1157. So now it appears things are pointing toward the upstream O2 sensor.

From http://engine-codes.com/p1157_honda.html#ixzz2smHdiUEh:

Possible causes
- Faulty Air/Fuel Ratio Sensor 1
- Air/Fuel Ratio Sensor 1 harness is open or shorted
- Air/Fuel Ratio Sensor 1 circuit poor electrical connection

Previously I wasn't getting any reading whatsoever from the upstream sensor, so at least now it's part of the equation (although I expected to still be getting the P0420 as well, but that's not the case).
 

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I would check the wiring to the sensor, if the wiring is good and the connectors are good sounds like you need a new AF sensor. I would use the Honda part which is around $125.

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Discussion Starter · #9 ·
So according to www.rockauto.com, DENSO Part # 2349040 is "OE Style" and it says that Denso is the OE Manufacturer of the part, so would this be acceptable (in your eyes)?



Like I said in my original post, money is super tight, so $35 in savings is a big deal but I don't want to end up replacing it again in 6 months if it's not the same thing.
 

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I would have no problems using the Denso part, it should work fine. I would not use a Bosch sensor.

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Discussion Starter · #11 ·
I would have no problems using the Denso part, it should work fine. I would not use a Bosch sensor.

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Got the Denso O2 sensor installed and the P1157 is now clear. However, after a few drive cycles the P0420 is back. So it looks like another $453 for the OEM catalytic converter.

But hey, at least now I know more about the emissions system than I ever thought I would!
 

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You still may not have a bad Catalytic converter, take a look at the freeze frame data and see what you long term fuel trim was when the P0420 code was set.

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Graphing the front and rear sensors will give you a good picture what may be happening as well.

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O2

For fun I ran another scan and now I no longer get the P0420, but I'm getting P1157. So now it appears things are pointing toward the upstream O2 sensor.

From http://engine-codes.com/p1157_honda.html#ixzz2smHdiUEh:

Possible causes
- Faulty Air/Fuel Ratio Sensor 1
- Air/Fuel Ratio Sensor 1 harness is open or shorted
- Air/Fuel Ratio Sensor 1 circuit poor electrical connection

Previously I wasn't getting any reading whatsoever from the upstream sensor, so at least now it's part of the equation (although I expected to still be getting the P0420 as well, but that's not the case).
I also got a upstream O2 code, 98 accord 4cyl. Rockauto had a OEM type (dont recall man.) for $20...but live in SC where theres no emmisions
 

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Even without required emmisions test I would fix this, a bad O2 sensor leads to a bad catalytic convertor.

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