Troubleshooting: (1) failure causes analysis causes both sides of the headlights are not bright causes: 1) protect the fuse of the headlamp circuit. 2) the headlamp controls the damage of the relay. 3) the light control switch is damaged. 4) headlamp wiring connection has bad contact or loose. 5) the filament on both sides of the headlights is burned. (2) the failure probability of the headlights is different, the probability of the burning of the headlights at both sides is small, and the probability of the fuse to burn out is relatively large. Therefore, first check the fuse and pull out the headlamp circuit fuse (10A) from the fuse box of the cab, and the fuse path is not burnt. The connection light of the front light relay is connected with the wire to control the switching terminal of the switch terminal to make the relay coil electrified. The result of the headlight lights up, indicating that the headlamp relay, the front light and the front light circuit are all without fault, and the fault is connected by the light control switch or its line. Check the connectors and lines at the combination switch. No loosening or breaking off is found. Pull out the combination switch connector, switch the switch on the connection position, and use multimeter resistance file to check the resistance of the light control switch terminals. The test result is infinite resistance, indicating that the light control switch is defective. (3) the fault handling measures dismantled the combination switch and found that the internal spring fell off, causing the switch contacts to be unable to touch. After the spring is reset, turn on the headlamp, the headlight is on, and troubleshooting.