It's not heating up. It tumbles. I already checked the vent and air is flowing. Also, ckecked the thermal-fuse and thermostat and their continuity is Zero. Also, the igniter looks in one piece. And turned the breaker on and off. What could the problem be?

Does the ignigtor glow? Is the burner coming on?

No, the ignigtor doesn't glow.

If the ignitor does not glow, nine times out of ten it is bad. Can you see a white or yellowish burnt area on it? If you cannot see a brek in it test for continuity. A bad ignitor will show no continuity and a good one will show 50-600 qhms. Test for resistance across the element