how do you tell when the FCA is bad

 

It'll idle loud. Meaning the injectors will get a little too much fuel, causing a little injector knock.

 

Unplug it and see what it sounds like!

 

Should idle rough, really rough. Don't let it run too long with it unplugged.

 

It will also cause the idle to slowly surge up and down sometimes...

 

Hard starts, slow starts, no starts, rough idle, surging idle, intermittent loss of power, surging at road speed with no load, low rail pressure, excessive smoke under heavy throttle, truck falls on its face for a few seconds then takes off, CC surge.

Just a few of the MANY symptoms.