Will a faulty o2 sensor with a heater circuit (4 wire sensor) show CEL right away when the engine starts or will it wait until it gets to the 600 Deg. temp. This would be on a 90 accord ex.

Hard to say...it would light the light when it sees a problem....if it disregards the o2 signal for a certain time period to allow for sensor warm up, then no, it wouldn't key on it right away...