I just installed a new exhaust from the header back with a test pipe and cat back...i reset my ecu because the check engine light came on when i moved the car with just the header on and when i drove it to my buddys shop (about40 miles) on an open header...now that i have everything on and reset the car at idle fluctuates from 1k to 2k and drops back to 1k and back up to 2k and keeps doing this...no CEL or any other issue..Any ideas

 

Does it do this only when the motor's hot or cold? Are there any times it doesn't do this? You sure the O2 is wired up properly? I don't suppose you checked the code?

 

there isnt a code...it o2 was direct oe replacement i just pluged it in..it does it all the time

 

This is the code I meant. Did you check it before resetting the ecu?

 

yeh it said catalitic over threshold

 

Check the grounds- thermostat ground, valve cover ground and battery negative strap. If they're ok, check the ignition, particularly the plug wires.

 

ok i lied it only does it when the engine warms up

 

It almost has to be an O2 problem. It's only happening when the computer is using the O2 sensors and that seems to be the only part you messed with that could cause this problem. I don't know if it's the sensor itself or just a loose wire but that's where you'll want to look.

 

iac?