So you don't have any codes currently?

You need a way to check your HPO pressure and you can do that at the ICP connector if you don't have a ScanGauge or something similar. It's possible that it's a HPO issue, but it would be worth it for you to find a way to monitor your HPO system while cranking.

Lacking a gauge - there is a way to use a volt meter to see what the ICP sensor is putting out.

If it turns out to be your STC fitting - you don't have to replace the entire pump, but you do have to remove it to replace the fitting.