JohnNH

This is a post I first posted on driveaccord, but I didn't get much response there in terms of what the expected "good" downstream O2 sensor signal should look like on a 7th gen accord. It seems that there are some forum members here who are very knowledgeable about catalytic converters and o2 sensors (from searching around other posts) and am wondering if anyone has experience diagnosing a bad cat from the downstream o2 sensor?

Here's the original post:

At 313K miles (2007 4CYL Accord SE) my check engine light came on. There was a single DTC:

P0420 Catalyst System Efficiency Below Threshold

Generally speaking this code indicates a bad catalytic converter, but I went through some diagnosis to try to make sure before buying an expensive replacement cat. What I found was interesting to me so I figured I would post it here in case it is useful to others.

First, I found that the often cited temperature test did not show any problems (as shown in the following IR thermal image):



Second, I found that the secondary oxygen sensor was not a constant output (even with the new cat and good O2 sensor) as it is on many vehicles. I did find that the lean/rich oscillations with the bad cat were much faster (1 to 2 second period) than with the new cat (10 to 20 seconds). So perhaps the presence of high frequency oscillations is a good diagnostic sign for a marginally failing cat. I would be interested if anyone else can corroborate this finding.



I replaced the cat (with an OEM replacement):



And that did indeed fix the problem (no codes after about 1000 miles and I passed inspection).

Many more details on the diagnosis and cat replacement are in the following PDF write-up if you are interested:

Updated Link 2/19/2023:
https://accordrepairsdiy.wordpress.com/

I would be very interested if anyone else has seen similar downstream o2 sensor waveforms with a marginally failing cat.

Thanks,
John

cj1

Right you are, pre to post cat ratio should be in the order of 1 to 10 as a minimum.

DCFIVER

It was a practice widely utilized in the early 90s. Now many cats are of the LOSC(low oxygen storage capacity) type and this test would be inconclusive at best.


Problem is there is no published data as to which vehicles utilize LOSC and which ones use HOSC cats, so that test method was abandoned.(by most professionals)The temperature test is also essentially useless.



Diagnosing cats failures are very simple. The modern PCM algorithm on cat testing is extremely precise. One simply needs to look at fuel trim, TSBs and the presence of a factory (or equivalent)cat. If those 3 things check out,and you get a cat code,the vehicle needs a cat.

Plain and simple.

DCFIVER

Says who?

JohnNH

Thanks DCFIVER, I was hoping you would post as I had seen your knowledgeable posts about P0420 before.

My case certainly confirmed your advice about the temperature test being inconclusive at best (in my case, the cat was indeed the problem, but looked "fine" in the temperature test).

One question, by "look at fuel trim" I assume you mean to just ensure that the fuel trims are reasonable low (under 5% or something) to be sure there isn't a fuel/air mixture problem fouling the cat?

I am also still curious if the frequency of the downstream o2 sensor oscillations might be a good way to gauge cat life, i.e. determine when the cat is about 50% life left, but that is not too important... Just a curiosity.

Thanks DCFIVER for your valuable contributions to this forum. I really value the fact that you have data to back up your advice rather than just passing along folklore...

cj1

OP-your findings are valid.
As you posted the O2 sensor oscillations were much slower with the good cat given other engine parameters were constant.

The 420 code itself is derived from pre to post cat ratios. This is obvious in your case.

I believe this can be useful to DIYs but as has been pointed out when a professional sees a P0420 and the other criteria( FT, OEM cat) is met the cat is going to be considered at fault.

DCFIVER

Yes the vehicle must be in fuel control in order for the cat to be effective. By monitoring trims you can see if there is an exhaust leak or other a/f mixture issues.
No. Cat codes on most manufactures set when the cat is unable to achive greater than 95% efficiency.

Again not knowing the type of cat and type of test being performed(there are 3 known testing methods in use right now) utilizing o2 oscillations would be inconclusive at best and lead to a false negative at worst.....

MAD_MIKE

Engine tune/condition is a factor in the P0420 being set as well. Verify the engine is in good tune before blaming the catalyst.
Something as simple as loose/worn plugs, faulty COP, or valves in need of adjustment may not initially set a misfire code but may set a P0420 code.

Remember P0420 is 'Catalyst System Efficiency Below Threshold'. Not catalyst needs replacement.

Always do further diagnostics before blaming the catalyst.

achonda06pilot

Hello, i needed to replace the same (rear) catalytic converter of my 2006 Honda Pilot LX (2WD) in less than 16 months. The first time it happened in October 2014 when the vehicle had 160K miles (Car's service code was P0420), and the second time it happened this month (same code P0420), when the vehicle had 184K miles. I have it replaced at Honda Service (Lute Riley in Richardson, Texas) both times.
When it happened for a second time so quick (160k miles wth first converter vs 24K miles with the second converter), I asked Honda Service for a free warranty replacement, but they said it only has 12K miles, and 12 month warranty on repairs, they called Honda corporate to see if they would do anything extra (they did not), but Lute Riley gave a discount to do it at their part & labor cost only ($800 first time vs $550 second time).

What could be the cause of the second catalytic converter to go bad so quick? I have been the owner and only driver of the vehicle since i purchased it new and my driving habits have not changed.

hondapro87

100% Correct