I have honda civic 2000 lx. I had a check engine light on, so two months ago I replaced exhaust manifold, converter, and O2 sensor. Check engine light went away. It is back after 4000 miles. I did not change the second O2 sensor which is down in the converter. What could be the problem?
Thanks

You likely need to just replace the downstream oxygen sensor. Find out, first, what code was thrown.