Either the ECU can't see the heater circuit working, or the heater in your o2 sensor failed. That's usually all it can be unless something is shorted somewheres.

If you need to hunt wires or pins from the bulkhead connectors or ECU pinouts... provided that you're still using the EG harness, there's a diagram on my... er... your old civic page in my sig.

Edit: THE LINK THAT'S NOT IN MY SIG.

EDIT X 2: The wiring diagram