I have a 2000 honda civic hatch and CEL is on. Its a P0420.
Everyone says to replace the CAT, but I took it to get tested and it passed. O2 sensors passed also.
I dont have any air leaks.
Ive changed spark plugs, wires, cap & rotor.
Still CEL is on.
I was gonna get the ignition coil tested...
Any help would be great.

thanks in advance

 

Stock exhaust? Mileage? O2 sensors or catalytic convertor replaced before? Made sure there is no exhaust leak?

Help us help you.

 

it's probably a cracked exhaust manifold.

 

You sure? How do you know?

.

 

The car is completely stock. Im not sure if anything has ever been replaced.
I took it to get smoged and it passed all the tests except for the malfunction indicator light test(CEL). btw i live in california.
I just got all the air leaks fixed. due to idle searching. Also cleaned the IACV.

 

117K miles on vehicle

 

Have you tried clearing the code?

 

x2^

not the same code but i had a bad 02 sensor that set the cel off, satyed on after it was replaced for about 3 months until i finally hooked up my scanner and cleared it and it never came back on

 

Ok ill clear the code tomorrow morning. Thanks
Ill let you know how it goes.

 

Just because an o2 sensor is bad does not mean it will fail emissions. It means you reading will be higher than normal, but it can still be within Cali's specs.

 

Exactly.

But just like anywhere else that has inspection, if a CEL (MIL) is on = automatic fail, which is what happened to the OP.

Luckily the P0420 code usually take a little while to pop up. I would clear the code immediately before showing up for the test.

..

 

Cleared the code and the light came back on within 30 miles.
Recommend changing the O2 sensors even though the passed?

 

Take the heatshield off the exhaust manifold and check for cracks, usually where the O2 sensor plugs in, can't remember.