I just recently installed a 4-1 header on my 99 civic ex w/ the D16. I also installed a Hi-Flow cat. I drove it for 2 days and then out of nowhere my check engine light came on. I have got both o2 sensors installed also. What is that caused but and what can I do about it. My buddy also installed a header on his neon and his did the samething. Modern performance told him to upgrade the Mass air flow sensor. Does that sound right.

 

o2 sensor

 

Same thing when i installed my headers. Yea it is the 02 sensors, my friend told me it's something to do with the stock sensors and how the second one has to read the air and it has to be richer or something than the air flowing thru the first sensor. Lol im not positive tho, haven't really fixed the problem yet... I keep resetting it at my friend's shop and it doesnt come back on unless I floor it or drive past 80.

 

It's because of your cat. Jump the service connector and tell us what code you are throwing. I'll bet it's 63 or 67.

 

Well I will let you know what it throws code wise when I get it to the shop. Thanks