I put headers in my 2002 honda civic but i didnt put the OEM and put the oxygen sensors on then i ran the car for a while drove about 30 miles then check engine light came i thought it was the oxygen sensor hook up the check engine scanner and the code came out p0420 mod OE 1/2 catalyst efficiency below threshold dose this have to do with the headers i put on do i have to put on the OEM ? THANK YOU IF YOU CAN TELL ME WHAT THE PROBLEMS IS

 

it means you have no cat because you removed it get a angled o2 simulator. california has very strict laws when it comes to this good luck with that.

 

Your spelling isnt very good, i get confused half way through.

 

if you put on Headers and didnt put the both O2 Sensors on then thats your problem, your missing 1 or both your O2 sensors and your throwing a code because of it

 

Did you buy the right header? LX / DX vs. EX ?

Can you take a picture of what it is you've done?


Either way, if you left an O2 sensor unplugged, that's the problem.



P0420 usually means your catalytic converter is toast. But if you had no problems before you installed the header... yeah.



EDIT: Do this..

1. Reset the ECU by either disconnecting the battery, or by removing the Number 6 fuse from the under-hood fuse box for a couple minutes.
- Then reconnect the battery, or reinstall the fuse.

2. Start the engine, leave it in park or in neutral and hold the idle at 3,000 RPM until the radiator fan comes on.

3. Drive for about ten minutes (highway or freeway) without stopping. Your speed can vary.

4. If you have an Automatic Transmission, leave it in Drive. If you have a Manual Transmission, leave it in 5th gear.
- Drive steady at about 60 MPH (50-62) for 30 seconds

5. Repeat step 4 three times. Between each repetition, take your foot off the gas for at least 2 seconds. If you have to stop for any reason, you'll have to repeat steps 3, 4, and 5.

6. Scan for temporary DTC codes.
- Does the scanner indicate Temporary DTC P0420?

Yes: Check and/or replace your catalytic converter.

No: Check for readiness code completion. If the code is complete, it was an intermittent failure and the system is OK. (at this time)
- If the readiness code is incomplete, repeat steps 2, 3, 4, 5.

 

I had the same problem with my 02 RXS-S and the ECU reset worked for me in fact anytime you mod your car you should clear the ECU and follow the specific steps. Helped me out big time after my upgrades.

 

it has both 0 2 sensors on the new headers. but i noticed that on the new headers it didn't have a cat on the headers like on the old one. do you think i bought the wrong headers? i'll put a picture so you can see.

thank you

 

Oh, you bought an aftermarket header, not an OEM replacement. Sorry, I didn't make sense out of that in your original post. Okay, that changes things a little bit. The aftermarket header doesn't have a built-in catalytic converter like the stock LX / DX header. That is the cause of your MIL (check engine light) so, as mentioned by lowlife9, you will need to buy an O2 simulator. Then clear your MIL by following step number 1 listed above to reset your ECU.

After you have resolved the code issue, you will need to put your car through Idle Relearn. It needs to be done any time you reset the ECU, or disconnect the battery.


Idle Relearn Procedure:
1. Make sure all electrical items are off. (wipers, heat/ac, radio, defogger, ... everything)
2. Start the engine and hold it at 3,000 RPM without load (in Park or neutral) until the radiator fan comes on, or until the engine coolant temperature reaches 194° (90c)
3. Let the engine idle for about 5 minutes with the throttle fully closed.

Note: If the radiator fan does come on, do not include its running time in the 5 minutes.

 

^how important is the Idle releran really?
I've never done this at any time and i've reset my ecu and had my battery d/c many times and my car is running just fine.

 

According to the manual, it's to establish idle characteristics in the ECU. Considering that it only takes a couple minutes to do, I've just always done it. (as I prefer to do things by the book, rather than cutting corners)

 

I wasnt cutting corner, i just never knew about this before untill now, lol
i will do this from now on tho.