My 83 Maxima has 212,000. I just replaced the rear three injectors.
The plastic electrical connectors are getting very tired. Like cracked
and chipping away where the metal "clips" hold them on tight. Can I
get replacement connectors , or a section of harness with new plastic
connectors etc? Also, how do I determine if the catalytic converter is
worn out/plugged up? Thanks

 

Replaced all 6 on my '82 not too long ago and had the same problem. I used electrical contact grease. it helps with the contact and helps hold them in place.

 

I dont' know if you can buy a new harness, but you could try looking for a 280zx in a junk yard or find one that is being parted out in your area. Inspect the plugs on that, and if they are ok, you can cut out the part you need or just use the end clips. Those are the exact same style as your Maxima, and a 280zx is easier to find.

 

Take your car to Nissan. Better yet, Call first. They had a recall for fuel injectors and fuel pump they will replace everything;
harness & injectors, for free.