When testing the cad cell with the burner on, the ohmmeter should drop to somewhere between ____________ to ____________ ohms.

When testing the cad cell with the burner on, a proper function is indicated when the resistance readings fall within the range of 300 to 900 ohms. The cad cell, which is a photoelectric device used to detect the presence of a flame, needs to exhibit low resistance when the burner is operational, as this signifies that it is accurately detecting the flame.

When the cad cell detects the flame, it allows current to pass through, leading to a lower resistance reading, which is necessary for the proper operation of the burner. A reading outside the specified range could indicate issues such as a malfunctioning cad cell or poor flame conditions, which may lead to burner shutdown or operational inefficiencies. This range reflects the expected performance under normal operating conditions, ensuring that the burner will operate safely and efficiently.