98 Honda Civic DX 1.6ltr. Check engine light on, code reads need new catalytic converter. Is it possible that the after cat sensor is bad, not the converter?

 

What's the code that you're getting?

The best way to diagnose O2 sensors and catalytic converters is too hook your car up to a DSO or even a laptop based scan tool so that you can watch the O2 sensors feedback.

To answer your question- Yes, there is a chance that it could just be a bad O2 sensors, however unless its been tampered with, I personally doubt it. If your O2 sensors is reading poorly, there is also a good chance that your cat could be bad as well.

My recommendation would be to first, diagnose the issue. Second, replace what needs to be replaced. Finally, FIND THE CAUSE. Just replacing the parts isn't going to fix the issue, you need to figure out why they went bad. One thing to understand is that CATALYTIC CONVERTERS DO NOT GO BAD. They will last the lifetime of a vehicle easily, however there things that kill the cat. A coolant leak, a rich/lean condition. This is what you will need to find if the cat is bad, or you'll be replacing cats over and over.

This graph shows the difference between a good cat and a bad cat.
The blue is your upstream O2 sensors, reading rich, lean, rich, lean- going into your cat. The red is the same reading from the downstream O2 sensors, which tells us that the cat is not working properly

 

Thanks for the input. I was going to replace both sensors anyway, when/if I replaced the cat. If it's the cats that bad, and I put in a new sensor, will it damage it?

 

nope.

 

Thats retarted to replace both o2 sensors and the cat just for the code. Either you get a decent scan tool so you can monitor the cat or take it some where.