i got a 89 honda civic 1.5 with a hollow out cat the ran good until i took it on the highway for like 3hour and now the car runs rough and im lossing power taking off but when it get up to like 3000 it runs good i just replace the muffler with a stock one and changed the plugs the car has 194000 on it it ran good until like i said took it on a trip any help will be helpful thanks and the check light comes on while on the highway

 

What is the code? It will be the blinking light on the ecu!

 

its a o2sensor i had the computer scanned

 

what would cause a engine to run rough like that