There is a well document startup sequence for a furnace. Once you get familiar with you, you can watch it go through the sequence and spot where it fails. Starts with the inducer motor.. And the final one is once the furnace starts up give it another couple minutes to ensure that the flame...
And what about all the inventory on your truck? Does the customer think the 50+5 MFD cap he needed was just the one you had on your truck? And what if the 40A/2P contactor was showing its age? And you just found it, saving him another trip+service call.
These days, the install part of HVAC is performed by close to unskilled labor. It's also pretty rough work.. especially the duct work part. All OK for a packaged unit. But soon you have a split system, then there a few steps that need to be performed thoughtfully. Especially with the 410A...
Like micromind said.. the Neutral on the Aux Heat side is irrelevant. The N just ensures the frame is grounded. The heating circuits are powered by 240 single phase - the two Lines you get into the house. If you are experiencing a voltage drop.. it is resistance in the line. Bad breaker is...
The part you missed is the licensing. You have to be EPA certified to handle any of this.. and on top of that you also have local building codes, which is a separate license, depending on your locality.