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1999 Honda Civic Oxygen Sensor "PO-135"

This code came up (PO-135) and when I pulled the exhaust manafold heat shield I found that the manafold was slightly cracked - so I replaced it (which includes the cat) and the OS (the one before the cat) and reset the check engine light with my code reader, but it keeps coming back on - what now?

I assume you mean code P0135. That code indicates there is a problem with the O2 sensor heater circuit. The heater element is either bad or there is a problem with the circuit to the heater element.

Here is a link to a little detail of what causes a DTC code P0135 to show up: http://www.obd-codes.com/p0135
Since there was a crack in the exhaust manifold, the hot exhaust may have damaged the oxygen sensor wires and connector. Examine them.
Here is a link to a post right here at cartalk: http://community.cartalk.com/posts/list/21/1735010.page It explains how to check oxygen sensor heater(s) (and the PCM control when a scan tool, or a inductive clamp-on amp meter is used).