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Discussion Starter #1
I have had the P0420 code show up twice now

Before I do any thing expensive I am going to pull out the ERG valve and clean it out.

the instructions for doing it where not all that clear to me.

My father in law (who is a mechanic) said I should change the thermostat as well.


The car has 148k miles and is an 2003

Oh, I change my trans and diff fluid every 30k miles. That seem resonable right? I do have a trans cooler, not really for towing as much as it is for piece of mind.
 

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Discussion Starter #2
what exactly is the EGR?


I cleaned out the ports ect under the top engine cover 6 months ago
 

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The EGR valve helps your car more efficiently and completely burn fuel by recirculating a portion of your exhaust and running it through the combustion process again.

Hopefully that solves the issue. It may also pertain to the catalytic converter.
 

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Discussion Starter #4
ok, I am going to replace the 02 sensor

But which one?

I need a diagram of the exhaust system. does anyone have one
 

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The P0420 code comes up due to oxygen sensor readings that are out of a specified threshold. This does not necessarily mean that there is a bad sensor.

Simply put there are 02 sensors in front and behind the cat. The pcm compares the two values and looks for a difference. They should be different which indicates that the converter is cleaning the excess hydrocarbons from the exhaust. The upstream sensor sends a signal to the PCM as a feedback for determining injector pulse width and ultimately air/fuel mixture. The 420 code is usually thrown to indicate that the converter efficiency is below threshold.
So you may have a bad o2 sensor, dying/dead cat (sorry this one doesn't have 9 lives), stuck injector, dirty AFM, or other issue affecting fuel mixture causing a rich mix which ultimately taxes the converter.
New 02 sensors and new converter will fix the problem, but it might only be temporary if there are other underlying issues that initially caused the converter to fail.

The EGR system is only in place to lower engine combustion temps which prevents NOx (oxides of nitrogen). It does this by allowing the inert exhaust gases to take up space in combustion chamber lowering efficiency and performance. The EGR valve has its own codes and may not be related to your issue.

The best thing to do before you start throwing parts at it and hoping is to hook it up to a real scanner and look at real time voltage readings from the sensors. Look at the spark plugs too.
 

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Discussion Starter #7
RallyD said:
The P0420 code comes up due to oxygen sensor readings that are out of a specified threshold. This does not necessarily mean that there is a bad sensor.

Simply put there are 02 sensors in front and behind the cat. The pcm compares the two values and looks for a difference. They should be different which indicates that the converter is cleaning the excess hydrocarbons from the exhaust. The upstream sensor sends a signal to the PCM as a feedback for determining injector pulse width and ultimately air/fuel mixture. The 420 code is usually thrown to indicate that the converter efficiency is below threshold.
So you may have a bad o2 sensor, dying/dead cat (sorry this one doesn't have 9 lives), stuck injector, dirty AFM, or other issue affecting fuel mixture causing a rich mix which ultimately taxes the converter.
New 02 sensors and new converter will fix the problem, but it might only be temporary if there are other underlying issues that initially caused the converter to fail.

The EGR system is only in place to lower engine combustion temps which prevents NOx (oxides of nitrogen). It does this by allowing the inert exhaust gases to take up space in combustion chamber lowering efficiency and performance. The EGR valve has its own codes and may not be related to your issue.

The best thing to do before you start throwing parts at it and hoping is to hook it up to a real scanner and look at real time voltage readings from the sensors. Look at the spark plugs too.
Thank you so much for that Reply.

I have 145k miles and the plugs where changed at 105k miles. Would looking at the plugs indicate an issue?

where do I get a real time scanner? I have one that reads the code number and description. I guess I need to send it off to a shop. Does it have to be an Acura shop?

Thanks again for the long reply, that really helps
 

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The scanner that you would need is expensive ($2000+) I had one a few years back but sold it because I didn't use it enough.

Looking at the plugs could still offer a benefit. They should look similar. If one of them is discolored, then it can help to narrow down the issue. I suggested this as it is free and I know how good it feels to figure out the problem before having to take it in to a shop. Doesn't hurt the wallet either, lol.

Doesn't have to be an Acura Dealer but I will usually try and find an independent specialist that typically services Honda and Acura vehicles. Obviously don't take it to Pep Boys or Jiffy Lube, etc.

Good Luck,
Derek
 

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Discussion Starter #9
RallyD said:
The scanner that you would need is expensive ($2000+) I had one a few years back but sold it because I didn't use it enough.

Looking at the plugs could still offer a benefit. They should look similar. If one of them is discolored, then it can help to narrow down the issue. I suggested this as it is free and I know how good it feels to figure out the problem before having to take it in to a shop. Doesn't hurt the wallet either, lol.

Doesn't have to be an Acura Dealer but I will usually try and find an independent specialist that typically services Honda and Acura vehicles. Obviously don't take it to Pep Boys or Jiffy Lube, etc.

Good Luck,
Derek
The acura dealer charges $55 to diagnose the issue. That seems reasonable.

They said it is most likely the Cat. So, can I just go to my local muffler shop and have them weld a new cat in? or does the MDX have some special cat?

I am seriously thinking about just getting one of these

http://www.o2simulator.com/index.php?main_page=product_info&products_id=9
 

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Ok then, I wasn't aware that you were willing to go that route. Let me help you save some money and not tap into the wiring.
Hows $5 sound?

My suggestion is to get 2 spark plug anti foulers from the Help! section of the parts store.
Dorman Motormite Help! #42002



Leave #1 as is. Drill 1/2" hole through #2 and install as pictured below. Reset codes and you should be good to go.

 

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No leaks. the purpose of the 1/2" hole is to allow the sensor to fully seat against the #2 part. Without the hole drilled, it would only screw in partially, creating a leak around the threads.
If there were a leak at that location, the sensor would produce more voltage due to increased oxygen content in the exhaust stream. This in turn tells the computer that there are excess hydrocarbons present when in reality there are not. I could get into more detail but its cheap and easy fix that can be permanent or temporary.
Basically it puts a space between the sensor and the exhaust flow with a ~3/16" nozzle on it. The result is a slower reacting rear sensor of which the readings fall within the acceptable catalyst efficiency range.
I would just put a dab of anti-seize on the threads so if you ever wanted to return it to original it would be easier, something that would not be possible with the O2sim.
 

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Discussion Starter #13
Thank you for all your help. I went to the parts store today, but they freaked me out about the cat backing up ect ect.

Anyway, the cat is not $1,000 like people where acting. I just found one for $240. It looks like it should be pretty easy to replace.

Easy with torch and impactor :)

So I am going to start with the cat and then the o2 sensor
 

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, but did you try replacing the O2 sensor yet?

While not an Acura, my wife's car recently gave a P0420 code due to a bad downpipe. It had come loose from the turbo, thus allowing an exhaust leak. You may find it useful to check for exhaust leaks upstream of the O2 sensors.

Good luck with your repair! :)
 

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2007 Acura MDX P0420

My 2007 MDX showed a P0420 code and I checked it online, it was a bad catalyst converter Bank 1. My mechanic told me that I have to replace catalyst converter Bank 1 and not the two O2 sensors problem, the catalyst converter cost about $900 at Acura dealer and is not including labor. Suddenly, I remembered that I had a recall from Acura on Sept 2011 about reprogram my MDX computer because of inject too much gas when it on star up cold and cause the catalyst converter to plug up. I went to the Acura dealer and ask the service manager about catalyst converter problem. He checked my record and show that my recall cause this problem and my catalyst converter still under warranty up to 120000 miles. he scheduled my car to replace catalyst converter right next day for free. After they replaced the catalyst converter, I felt that my car have about 15% more Horse power and it is smooth when drive on the freeway.

Everyone who own 2007 -2009 MDX should check with their dealer about the recall and you have to do the recall to qualify this extend warranty for catalyst converter.
 

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did my yearly maintenence, including spark plugs..so it doesnt have to do with spark plugs. if your plug weren't firing you would definitely know by vibrating or driving .then 420 comes up .. So, for someone with 0420 code on 2003 mdx? it's most likely cat convertor replacement? anyone know how much the part was and cost at dealership? i don't think going non oem on a cat convertor would be reliable.
 

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Older thread but just wanted to acknowledge the suggestion from RallyD. Clearly a knowledgable contributor offering an option that may not appeal to some but does to me. Thanks RallyD.
 

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Not to beat a dead horse, but if you haven't checked the O2 sensors, do that before you spend the money on a cat. Much simpler repair.
 
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