If the laser tube won't fire the laser and the ammeter reads a value higher than 4 mA, the laser tube is faulty. Replace it with a new, identical one. 

If the laser tube won't fire the laser and the ammeter reads a value lower than 4 mA, 

1. Connect the wires of the cathode and anode of the laser tube (as shown).
2. Press the "test" button on the laser power supply.
3. Check the ammeter reading.

• If it reads about 8 mA, replace the laser tube with a new, identical one.
• If the reading is still too low, replace the laser power supply with a new, identical one.