I have a 2006 Honda Accord Lx 4 cylinder. I've been having some issues with it for the past couple months. I changed the alternator a little over a month ago and it was doing fine for a while. Then, a few weeks ago, it started to shudder pretty violently when I went to park it. I checked fluid levels later, and they were all good. I started it back up and it was fine. Then, it started having issues when accelerating to higher speed (around 60mph) and the check engine light would start blinking. So, I took it to a mechanic and they changed out the spark plugs and knock sensor and told me my battery was bad. Went and got a new battery and all seemed well, but then it would start shaking every now and again when started it up and the check engine light came back on. So, I took it back in and they ran the codes and said it was a code for the catalytic converter. They cleared the code and told me that I have any more issues, they'd look into a replacement catalytic converter. While I was driving, my car's check engine lightstarted blinking, again, and when I parked, it started shuddering, again, so I used an OBD scanner I got and the codes p0420, p0302, and p0303 came up. I talked to my dad about what was going on and he said he'd never heard of a catalytic converter causing a car to shake, so I'm wondering if anyone could give me advice as to whether I should get the catalytic converter changed or if there's a more likely cause for my car's symptoms?

 

The misfires are bad for the catalyst which is why the light blinks. That tells you you are ruining your cat.
Need to swap some things around and see if you can narrow it down.

 

Have any of you checked out Rock Auto

I got both my catalytic converters, along with both O2 sensors, MAF sensor and engine intake hose for around 650 parts and tax. Shipping cost aren't outrageous, unless you pick the quicker delivery options. I paid like 115 for shipping

It's my go to site for all parts and they have a wide variety of brands and every part you could ever dream of ... Just saying