put a new heard uppiipe and down pi-pe on my car today when i was done i went to fire it up and it fiored for a seconde then shut off and will not fire up now it cranks over fine but thats all it will do what could it be ?

all i have the back o2 sencore unpluged becues i have no cat to plug it into

try plugging it in, never know, it might work.

Does it just not catch? I'd check the plugs and wires. Something might be loose.