I have a 2002 Accord SE Coupe. It's got the F23A4 motor with 165k miles and I'm having some problems with performance. At low RPMs, there is low but adequate power. It doesn't jump off the line but it's (barely) drivable. Once I get past 2500 RPM or so, the power seems to taper off and up past 4000 RPM and there doesn't seem to be any power left.

Put differently, it will gradually come up to freeway speeds but there's absolutely no passing power. The symptoms don't seem to change when the engine is cold or warm.
Here's what I know so far:[ul][*]OBD-II code P0420 is set and keeps coming back after clearing the codes[*]The loss of power didn't immediately follow the P0420 code. It didn't really start losing power until at least 500 miles after the code first showed up.[*]I found a very small leak on the exhaust manifold. It's by the #3 cylinder up near where the tube is welded to the mounting plate.[/ul] Here's what I've done in the recent past:[ul][*]Replaced distributor cap and rotor[*]Replaced spark plugs[*]Verified spark plug wire resistances within spec[*]Adjusted valve clearances[/ul] I expect to have to replace the exhaust manifold because of the leak but since it's up near the cylinder and not after the O2 sensor, I'm wondering if that could cause the P0420 code.

I'm also wondering if the loss of power can be attributed to the exhaust leak. Or maybe the catalytic converter is blocked. There's no discernible rattle. Is there any way to easily check this short of removing the converter and visually inspecting it? If I have to replace the converter, should I replace the O2 sensors as well?

I have an Actron CP9180 scanner that can record data. I recorded a dataset while driving home the other day that you can download as an Excel file. I can record more if that helps the troubleshooting. I did notice that the second O2 sensor seems to fluctuate a lot more that it should, which I believe is expected with this trouble code. If there's any other OBD-II data that would help with troubleshooting, please let me know.

 

from what you described it can be a bad converter(hence the code) that is restricted (hence the lack of power).

do you have a way to measure the temperature before and after the cat? there should be about a 100F difference. this is a quick check. you don't have to replace the o2 sensors with teh cat.


we just had a guy with a clogged cat. it was easy to diagnose because it came in glowing red! so try and hold the rpm at 3000. if you look under and it's glowing, replace the cat.

 

I don't have an infrared thermometer so unless OBD-II shows the temp of the oxygen sensors then I'm going to have to try the glow method. Thanks for your suggestions. Any other thoughts?

 

A few more related questions.

What would cause the converter to go bad? I'd like to avoid putting on a new one only to have it go south soon after.

Any recomendations on the OEM vs. aftermarket vs. high-flow for the replacement?

 

well, a converter usually goes bad because reactants inside either get get coated with fuel or some other product(silcone, etc) or wear away/wear out.

they can wear away/melt or wear out very fast from a misfire. a very lean or rich running engine can cause premature death as well. can your scantool look at live engine data? look at the fuel trim - how is it? typcially if it doesn'tset a p0171 or p0172 it's ok, but if the LT FT is 1.17 or .86...yikes. it should be 1.00 in a perfect world.

the o2 sensors don't have a temperature sensor, they just measure oxygen content. you can buy an inferred temp gauge for very cheap at menards or home depot for pry about $50.

typically oem cats will last just as long as your original. however, teh cost is high. your aftermarket one may last longer or only 1/4 the time but it can be1/3 the cost or cheaper.


keep in mind the only way the cat would cause engine problemsin this case is if it's restricted. the temp test is just a quick check really. the best way is as you said - remove and inspect to see if it's causingyour engine's lack of power.since it's giving p0420 you might as well pick up it's replacement anyway. pop it in and clear the code. go for a road test. if the check engine light isn't flashing that means there isn't a misfire going on that's great enough to damage the cat. see how it feels. if you have no power still, let us know and we'll fix that #^@#.

 

I can look at live engine data with my tool and the LF FT is at -13.3%. I'm not sure how that correlates to the figures you listed.

Additionally, during a the period I recorded, ST FT was bouncing between 0% and the -10% to -20% range.

None of this has been sufficient to set any codes. The only one I'm seeing as a set or pending code is P0420.

 

I'm wondering if running the same set of plugs, distributor cap and rotor for 165k miles might have led to the operating condition that would damage the converter.

I recently bought the car from the original owner and found out that the guy hadn't had ever it tuned up. Since I already replaced those parts, it would be great to know that they were the cause.

 

0420 is a catalyst deficiency code. it is likely the cat as HondatechAV6 suggested (he is a Honda tech after all). Some people have tried replacing the o2 sensors without replacing the cat, but the code usually always comes back (0420 is not an o2 sensor code anyways.)
You could take it to a shop to test the cat if you want to test it out before throwing parts at it though...

 

That leak in the exhaust manifold can cause your O2 sensors to misread the oxygen level in your exhaust and lead to an incorrect fuel/air ratio. I would replace the exhaust manifold first, reset the ECU, and see if you gain any power. The cat could have been damaged during this process, but I would fix the part that you are sure is broken before you start throwing parts at the car.

EDIT: P0420 isn't a problem with the O2 sensors, just the numbers that they are reading.

 

If the leak is before the O2 sensor, wouldn't you expect the reading to be overly lean, causing long term fuel trim to go positive?

In my case, the long term fuel trim is going negative, indicating that it sees a rich condition.

Another question, how much of leak would you need to see. I put a shop-vac on the tail pipe and reversed the hose so it was blowing. With this setup, I could feel a very slight movement of air around the top of the exhaust manifold on the pipe for the third cylinder. Is any leak enough to cause this kind of problem of does it have to be somewhat substantial.