I've got my 2004 Honda Accord Coupe V6 6MT last December. If I went on a longer drive (10+ km), then the CEL would come on after a while. But it would go away after 3 driving cycles (i.e. engine cooled off before restart). It would never come on when I just went to work & home, which is 7.7 km one way.

So I got it scanned and it says P0420. Now I know that this is a very generic code; but I wonder if anyone ever experienced similar symptoms, i. e. that it comes on only on 'longer' drives and goes away after 3 driving cycles (and never comes on when driven less than 8 clicks at a go).

 

that code is usually a cat converter going bad

 

Thank you for your response.

I understand that P0420 is about cat converter inefficiency; what I don't understand is why does the CEL go off by itself after 3 driving cycles? If the cat converter is busted (or the 2nd O2 sensor is bad), wouldn't the CEL stay on?

 

When a malfunction is detected during the first drive cycle, a Temporary DTC is stored in the ECM/PCM memory. If the malfunction
recurs during the next (second) drive cycle, the MIL comes on and the DTC

The MIL will be cleared if the malfunction does not recur during three consecutive trips in which the diagnostic runs.

 

Thank you again.

Then this is an intermittent problem which happens only if I drive more than 10+ km at low, i.e. 2,000 RPM in 6th gear.

My problem is: I can get the cat converter replaced; but if there's something wrong with the engine (the exhaust "fouls up" the catalytic element some way, but not always), then I went to considerable expense without solving the root cause.

I guess I just have to take it to a very good mechanic.

 

It turns out that the CEL comes on only if the engine is revving around 2,000 RPM after some (10+ km) driving. I did my usual joyride without shifting into 6th gear; in 5th gear I was going around 2,800-3,000 RPM and the CEL wouldn't come on. But toward the end of the drive, when I slowed down and was doing around 2,000 RPM in 4th gear, the CEL popped on.

I took it to a shop; there's no hole in the exhaust or the cat. Mechanic told me that probably one of the O2 sensors is going ... but I shouldn't worry about it, bring it back if/when the CEL is on continuously (doesn't turn off after 3 drive cycles) and they'll figure out which O2 sensor has to be replaced. So that's what I'm going to do.

 

Your cat and sensors are fairly inexpensive for a Honda.

 

Time to find a new mechanic,the one you are going to now does not understand the code set criteria nor what is occurring.
Since OBD2 is almost 20 years old now,there is no reason a professional in this field should be telling you that nonsense about your O2 sensors in regards to a P0420 or P0430 code. There is plenty of training and information available to him.


You in all likely hood will be needing a new cat(s) Honda cats typically last around 100k-150k miles. They simply wear out due to heat cycling and vibration. Once the efficiency is deemed to be below %99, a code will set.


A good mechanic will understand how to determine if your cat is bad or if there are other issues.(A bad O2 will NOT be one of them)

 

Thank you. Still, there's no explanation why would the CEL pop on only in case of a particular set of circumstances: 10+ km drive, ~2,000 RPM. (It doesn't happen if I keep the RPM above 2,500).

 

Perhaps the issue is not the cat at all,but an electrical issue.


A proper diagnosis would be needed to determine the actual fault......