Check the battery voltage. (Low beam will be on when the battery voltage above 9V.)

Check the fuse and relay.

Check the polarity of ballast. (If the polarity are changed, low beam doesn't lighten)

Check the bulb connector securely.

Visually bulb checking (no filament): damaged glass, damaged for upper parts and lower parts of glass tube.

After 1~5, replace the ballast and the ignitor (ballast assembly).

Pull out the head lamp relay (Low) (A) and head lamp relay (High) (B) from the engine compartment relay box.

Check for continuity between terminals. There should be continuity between the No.1 and No.4 terminals when power and ground are connected to the No.2 and No.3 terminals.

There should be no continuity between the No.1 and No.4 terminals when power is disconnected.