Since you take the exhaust manifold down and the misfire goes away that means you don’t have the oxygen sensors hooked up, mainly the AF sensor which helps the computer regulate fueling. It may be that your AF sensor is bad.
If you have a way to look at data from the car, look at the short-term and long-term fuel trims as well as misfires, or it’ll be listed as roughness, for each cylinder to see if it’s misfiring on the other three but only setting a fault for one.
And I sent you a link for a FSM so you could have the specs for the sensors.
And I put the link up to the p0420 diagnosis thread because I’ve listed out all the things that will cause this code to trigger and it not be the catalytic converter.
Battery cable appearance can be deceiving. You have to put a meter on them. From the post to the end of the cable should be zero ohm. From the negative post to the alternator case should be no more than 0.01.
