Oxygen sensor question

[Russell]

Just the other day I reinstalled HS headers and on the gasket between the
primary and secondary added some Permatex high temp Ultra-Copper "sensor safe"
silicone gasket sealer along w/ the metal gasket.  I used a FelPro gasket
between the head and primary.  The check engine light came on after about
15min.  So I reset it and it seemed to go away.  Then after a 500 mile road
trip the CEL comes on again when I started the car again.  I just drove with
it on for the 500 back and it ran great with almost 36mpg.  After resetting it
again and the code was 0303 for 02, it just keeps coming back on.

I'm wondering if the Permatex Ultra-copper is what's likely causing this,
after all it's supposed to be "sensor safe"?  I've had it set off before due
to some liquid wrench, but it was fine after the stuff burned off completely.
Only other suspect I could see would be a bit of anti-seize.

Russell
'95 200SX