i have a 99 civic , the p0420 is haunting me. i replaced the cat (magnaflow), o2 sensors n still the check engine is coming on and it comes on when im on the freeway. i narrow my problem to the computer or the o2 sensor sub harness , does any1 have any other ideas? and where is the sub o2 sensor harness? i heard by the firewall, does any1 have pictures or a diagram?

 

i beleive that error code is because the 2nd o2 sensor after the cat is reading your emissions levels higher than what they should be. usually if the HCs COs CO2s or NOX readings are too high you might need a tune up. i'm sure your cat was fine. Hydrocarbons are caused by unburnt fuel, Carbon Monoxide can be affected by timing or vacuum, nox can be from incorrect fuel ratios.
i'd guess it just needs tuned up.

 

That's likely your problem right there. Inexpensive aftermarket catalytic converters don't contain enough of the precious materials required to process the exhaust gases. The catalyst monitor in these cars is pretty sensitive and the cars tend to be picky about converters that will work. With exception of a very select few, most of the aftermarket converters that I've dealt with will only last anywhere from 6 months to 2 years before the P0420 comes back. It's not an oxygen sensor fault (those will throw their own specific codes), and it is common for the low catalyst efficiency to be detected during freeway driving.

If you want the light to stay off for good (meaning in this case at least 8-10 years) you will have to pay up big bucks for an OE Honda converter.

There are also other ways to eliminate the code, but they are not legal and will not pass a visual emissions equipment inspection if required in your area.

 

Absolutely ditto that.

 

i have done a full tune up, i also cleaned the fuel system and cleaned the vacuum lines too. i used synthetic oil. and the check engine light still comes on . i also checked my distributer for any lose wires. i think is the computer or sub harness wiring for the downstream o2sensor

 

Doing anything other than installing an OEM Honda converter is just pissing into the wind. For the money and time you've spent replacing and servicing unrelated items, you could have had the problem fixed already.

Assuming EX trim level, you can purchase a Honda cat for a heavy discount at $614 + shipping from an online Honda parts wholesaler. List price at your local dealer will be a little over $812.

 

but magnaflow is one of the best cats , i dont think dat makes any difference but im gonna take it to honda deleader and see what they say. any1 else has a clue what it could be?

 

Buying aftermarket isn't pissing in the wind. I've kicked this around for a few years and it's still cheaper to buy 3 magnaflow cats rather than 1 OEM. Plus, most engine issues that burn out cats are themselves expensive to fix (oil burning). If you cant fix the engine issue then it's far better to burn out a $200 cat in 2 years rather than a $700 OEM cat.

It doesn't really take much time to replace one either. Five bolts is not a big deal. I did it in my driveway.

I also noticed significant power gains and increased mileage when I replaced my OEM cat with magnaflow.