***1999 Accord 2Dr. V6--I think it's an EX (whatever the Gold Package is, that's the one), I have 109,000 miles on the car***

I failed smog on Saturday and I need to get my car registered ASAP (my tags are already expired). I recently replaced the catalytic converter, but still had the check engine light on the night before. I went to AutoZone Saturday morning, cleared the code, and headed to the smog station (I am aware that you need to let the car run a complete cycle before assuming everything is fine, but I anticipated that I would fail the smog, and felt it would be beneficial to know which area I would fail in. That way I could determine whether or not the shop that installed the cat. was responsible).
The NOx levels were too high (avg. score is 43, max is around 400, and my score was above 600), all the other emissions levels were fine. I have read that high NOx scores can be attributed to the EGR. I took the car to my mechanic yesterday and after running diagnostics, he told me that the EGR was running fine, the catalytic converter was fine, and the oxygen sensors were OK.
He suspected that there was an ECM problem and that the computer was not communicating properly to the EGR. He told me that his shop wasn't equipped to take care of computer issues and that I should take it to the dealership, but I don't want to pay an arm and a leg for what I think may be a simple solution. I have been online searching for the answers to this probem, but have come up with little to nothing. I have heard that there may be a faulty fuse or maybe a wiring problem. I have noticed a short in my stereo system (sounds goes in and out intermittently) and my odometer bulb is out. These may be completely isolated issues, but I thought giving as much information as possible would be helpful and troubleshooting the issue.
Thank you in advance for any assistance (and I apologize for the lengthy post).

 

I would suggest that you clean the EGR passage, I think on that car you'll need to remove the intake plenium and find the port. The EGR valve itself might be working okay but if the passage is plugged up with soot it restricts the flow of the exhaust gasses. P0401 indicates the passage needs to be cleaned out.
here is a link to help you ;
EGR Port Cleaning

3. OXIDES OF NITROGEN (NOx)

NOX is the catchall name for a group of chemicals of different Nitrogen and Oxygen combinations. When enough NOx is present in the air combined with sunlight forms irritating smog when mixed other elements in the air.

Nitrogen (N) makes up approximately 78% of the air we breath and the rest, approximately 21% is oxygen. The engines in today's vehicles use oxygen to operate and exhausts the nitrogen. NOx is formed when the combustion temperature exceeds 2500° F.

Unfortunately an engine designed and tuned to produce low CO and HC produces high NOx The hotter an engine runs the more power it delivers, the more NOx it produces.

Late Model vehicles operate with a very lean A/F mixture so the NOx is very high and a "three way Catalyst is incorporated converting CO, HC and NOx.

A device called the E.G. (Exhaust Gas Recirculation) Valve was designed to lower the combustion temperatures by recirculating exhaust gasses back into the combustion chamber.

NOTE:
There is little the mechanic can do to control NOX emissions, except to make sure the engine and E.G.R. valve (on vehicles so equipped) are functioning properly.

One of the keys to emission control techniques is controlling the air/fuel ratio. The point at which the A/F mixture burns efficiently for peak catalytic converter operation is called the Stoichiometric Point or ideal air/fuel mixture. At this point the HC, CO, O2 is near peak. The point at which complete combustion occurs is 14.7 to 1A/F ratio (called "Theoretically ideal").

 

P0401 on a 99 v6 the intake manifold needs to come off to get to the port.

P0420 is a bad cat.

 

Thank you for all the help...the mechanic I went to said that the EGR was running fine, but I doubt he took the time to take a look at all the ports. Also, could the 420 code be coming from the 401 code (for example fix the EGR and the Cat. code might go away)? I ask this because I replaced the cat. converter very recently and doubt that this is working improperly. Thanks again.

 

Did you get it figured out?

I have a similar problem....had the EGR code. Took it to a local mechanic. $150 to repair. Now I'm getting the Cat code.[:@]

 

How many cats are on the accords (one or two)? Because i recently replaced the cat and clean the egr ports and i am get the 420 code.

99 accord v6 cp

 

I need a convertor but the dealer told me it was $1,000

 

Only one cat....yeah. OEM runs $700 at the dealer and they charge about $300 to install.
Mine is intermittent....comes on, goes off.... I'm going to chance it, if my Accord sells.