Resistor R4 failed open: LEDs 1 through 3 always off, LEDs 4 through 6 always on (with any substantial input signal amplitude at all).

Solder bridge (short) past resistor R3: LEDs 2 and 3 always energize at the same time.

Resistor R11 failed open: LED 4 never lights up.

Zener diode D1 failed shorted: All LEDs light up together with any substantial input signal amplitude at all.

Schottky diode D2 failed shorted: None of the LEDs ever light up.