Unplug your 350P before performing any maintenance. A maintenance issue may be causing your welder to malfunction. While you do this, quickly plug in a known working device to the same outlet to ensure the problem doesn't lie with the outlet.
Miller recommends inspecting the welding gun and weld cable for damage or cracks every three months, as well as cleaning and tightening weld terminals. Every six months, the company advises you to clean inside the unit.
Plug your unit back in and turn it on. Use your multimeter's tongs to check that your 350P's torque is properly set. The torque bolts are in the rear of the unit, in the upper right corner. With the positive lead against the right screw and the negative lead against the left, make sure the unit is putting out about 17 inch-lbs. of torque.
Familiarize yourself with the display unit's HELP codes. Page 46 of the manual in the Resources section will help you decode these indicators and identify your needed remedy. HELP will flash when a problem is occurring in the system, next to a number from zero to 11. For instance, if "HELP 9" is on your display, your Pulse MIG function is inoperable and you may need to replace your welder's pulse board.
Screw in all bolts and screws visible from the outside of the Millermatic 350P every three months as part of regular maintenance. Machine vibration can often loosen critical components.
Determine what your machine can and can't do. This will help you fine-tune your troubleshooting to identify the particular problem you face. The manual's troubleshooting guide on page 48 will allow you to address your specific problem with the proper response--whether that be adjusting certain settings or replacing a part.