K2633 troubleshooting

Good day

Relay 4 on the k2633 board is not switching. LED4 comes on when shorting input 4 to ground but seems dimmer than other LEDs.

Is there a way to troubleshoot faulty component (i’m thinking the transistor, relay or maybe LED) without removing it from pcb or should I just replace all components?

Check the board for mechanical problems such as shorts/bad solderings/forgotten solderings etc…
Check all resistor values
Replacing the transistors is the fastest and most reliable way to troubleshoot the active components.