Phizy

Hi. I have a 2003 honda civic. Here is a brief history:

Over the summer I heard an intermittent "tsch tsch" sound coming from underneath. Doesn't always occur, mostly when hot. Car does burn oil slowly.

1 month ago, got a P0420 code. I figured it was the cat going bad. I was ready to change the cat (perhaps based on some bad advice, I don't know). Engine light went away, decided not to change it (returned it).

Since then, I put in high octane fuel with SeaFoam to help clean things. 2 gas tank fulls. During the 2nd, the engine light came on again and now has stayed on with the same P0420.

Decided to drive anyway. Noticing possible power loss (maybe, it could be my paranoid imagination from reading too much).

I do have a spare O2 sensor (one on the cat).

I know what the description of the P0420 is.....I have read what it could be, what it 'probably' is etc. etc.

My question is this:

What is the next diagnostic step I should take?

My first thought is replace the O2 sensor on the cat since I have it. Other than that, what is the next best step?

Thanks.

ezone

Figure out if the cat was KILLED by misfire or overfueling.
Fix those if necessary.
Even a brand new cat can be killed in a matter of minutes...

Otherwise.....

Computer must have proper fuel control. Be sure of that.

Check for exhaust leaks between the engine and the cat.

If none, then replace the cat.
Factory will last a lot longer than most aftermarket units....unless something else kills it.



EDIT: On the power loss thing, you're probably just paranoid.

Phizy

Thanks. How do I go about doing those things?

ezone

Well, when I get one with a cat code and I know absolutely nothing about the history of the car, all I can do is look at possible likely causes and mileage.

Are there exhaust leaks that could have caused a "false fail"?
Is there a ton of miles and the cat died a natural death?
Was there a misfire problem, or any other running problem in the past that could have killed it?

IDK.

Is there a current problem that would cause a new cat to fail far too soon if I DO replace it?

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Does the engine run properly?

Is there a problem with misfire now?

Are there any exhaust leaks between the engine and the cat?

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Does the computer have control of the fuel?

On a scantool datalist: are the fuel trim values within normal and acceptable ranges under normal driving conditions? I'm mainly concerned with trims, but I look for any other anomalies too.

On a 5-gas analyzer (emissions testing areas): are pre-cat readings within normal ranges?

Phizy

Well....
It has 185k miles on it. Blew a head gasket last year.
Exhaust smells normal.

ezone

So it's got high mileage. Cat died a normal death I guess, as long as there wasn't any recent running problems.

Ummm, ok...
Kinda subjective, don't you think?
What good did that do?



Either throw a cat at it and hope for the best....

...Or check into the things I wrote up there, address any issues found, and then install a new cat with some confidence that it will live.

Phizy

Yes. I understand. I'm asking, how do I go about doing those diagnostic checks?

ETA: Bringing it down to Meineke for diagnostics...no charge.

Phizy

The issue is now potentially resolved. Updating for future searches.

Here is what happened.

184k miles on the car, P0420 code.
O2 sensors were fine, cat substrate was starting to break, partially clogged, and started to melt in places.
The "tsch" "Tsch" sound was due to minor, intermittent rod knock due to a drop in oil viscosity. The drop in viscosity was due to insufficient oil changed due to following the manufacturers schedule. The drop in viscosity caused combustion/leakage into the exhaust thereby fouling the cat. One problem led to another.

Solution: Oil changed from 5w20 to 5w30 and will be changed at more frequent intervals (3000k miles). The car will also be checked for oil and topped off regularly to prevent loss in oil to reduce further wear on the bearings.