No heat. Cycled system. Igniter would energize, flame would light, then system would drop out. Cycled several times with similar results. Removed burner box and gas manifold. Checked igniter, tested normal. Checked low voltage. Correct voltage. High voltage was tested. Correct voltage. Cleaned and checked flame sensor. Flame sensor was very corroded and dirty. Re installed burner box and flame sensor. Cycled system. System fired normal and continued to operate normal for over 15 mins. System is in major need of maintenance to help ensure proper operation. System operating normal at this time. Job complete.