Have a Miller Dynasty 300 that won't tig weld, displays HELP 12 code. Suggestions??

switch from tig welding to mig by switching your current from AC to DC for mig. change from your tig gun to your mig gun. turn water flow off. proper ground and weld away.

check earth cable and cleanliness of parent metal?

It's likely that your pedal is the problem. Pull the cover and check it electrically - these have been problematic in the past. You can further troubleshoot by selecting constant output and test the output.

Wow what a great machine. You have so many options that you should be able to control the ending arc. Yes, you should be able to adjust the intensity of the HF. Also if you use the Sequencer Control, you should be able to finish the weld in a slope manner. I am including the owners manual. Operation starts on page 38. Miller has a team of welding engineer that will be able to help you set up your machine. Wish I had one.

On this page enter your serial number and you will be able to download your manual http://www.millerwelds.com/service/ownersmanuals.php In the Dynasty 350 manual I see no "Help 12", there is a 32 and 22. If the Help 12 is only displayed when you push a button, it is saying the combination you have selected is not valid.

Have you turned it off and try blowing compressed air on the cooling fins?

It sounds like you need to change out your SCR chip on your motherboard; Miller makes one that i have installed on all our 350xl's, I don't have the number on hand but i can get it for you tomorrow.

proper polarity?

must use remote output for tig

From the user manual for a generic dynasty model welder:
2 Help 2 Display
Indicates an open in the thermal protection
circuitry located on the left side of the unit.
Contact a Factory Authorized Service
Agent if this display is shown.

So you could have anything from a broken or disconnected wire to a bad thermal switch.

You can get user manuals specific to your welder at www.millerwelds.com

Rank this answer please,
Carl