I just installed a 2/5" test pipe on my 99 Civic Si and drove it home over 100 miles. I drove it around town and all of the sudden I threw a CEL. I reset the ECU by pulling the backup fuse and it satyed off for another day. It has now come back on and I was wondering if anyone knows what could be the problem. The test pipe has some catalytic converter catalyst and the O2 bung so it shouldn't throw a CEL. Can someone please help me ???

 

the test pipe is not a test pipe if it has some catalytic converter catalyst so you will thought a CEL light everytime but to check there is a green plug under the glove box near the ECU, the green plug will have 2 wires coming from it going no where. so turn off the IGN. use a wire to conect the two wires. turn on the IGN nd look a your engin light long flashes mean 10X and fast ones are 1X
ie long, long, short,short, short, short. would be code 24
do a search on CEL codes to find out what they are i hope this helps