If the torch on your MIG welder will not light, but the "machine on" LED light is lit, first check the "safety" LED. If it is lit, press the "safety reset" button. Also, check the air supply to the unit. The machine cannot start without any air flow. Low air pressure will also cause it to not start. Set your machine to purge mode to turn on the air. The regulator needs to be set to a minimum of 60 psi. Raise the pressure slowly until the "air pressure" LED lights. If it still does not light at 75 psi, you must have a leak or clogged air filters.
If you cannot establish an arc with your MIG welder, even with the air flowing and the "output on" LED lights being lit, check to make sure the torch consumables are in tightly and not dirty or greasy. Also, check that there is a high frequency at the spark gap and that the CR2 engages. You may want to blow compressed air on all the upper compartments. If your arc starts but sputters, ensure the air supply is not being compromised by oil or a great deal of water. If there is either, you will need to filter the air or switch to nitrogen or bottled air.
If the "thermal" LED light is lit and the "fault" LED is blinking, it means the machine is overheated. You should let it cool and then reset it. Don't allow the air intakes to become blocked. If your "air" and "fault" LED lights are alternating, you need to replace the control PC board. If the "output on" and "fault" LEDs blink alternatively, the pilot arc duty cycle has been exceeded. Your machine will cool down and the lights will stop flashing in approximately 20 seconds.