I have a 2006 Nissan Frontier 4L, 2-wheel drive. One day two weeks ago I had a couple of codes pop up. One was a knock sensor code, the other one was a missfire on cylinder 5, and a catalytic converter problem. I thought the knock sensor would be easy to get to. So I pull off the intake phlegm. I got all new plugs and coil packs. Remove the phlegm and didn't realize that the knock sensor was underneath the intake manifold. Went ahead and removed the intake manifold and found that a mouse had eaten through the wires of the knock sensor. Rewired and replaced the knock sensors, then replaced the plugs and coil packs got everything together Still showed a code for the catalytic converter. After going to a shop nearby and was told it would be about $1,000 I said, let me try this. Ordered an aftermarket catalytic converter. After some problems finally got it in. I was so happy, reset all the codes. No problem. Smooth idle, smooth pretty good acceleration in the city. But when I got out onto the highway, I had good acceleration up to 3000 RPM then slight coughing and sputtering. And the check engine light started to flash but after I got on to level ground the flashing one away. After playing with it I found that everything was smooth out to about 3,000 RPM. Got home and checked my codes again. Now I have a p 0300 code, stating I have a random misfire. I'm kind of at a loss, any ideas?