and take it home to fix.

let us take a scenario:
you make this "diganostic chip" and it finds that there is an analog disrepancy and right there in the mixing plant you take your little soldering iron and replace the a/d chip as the diagnostic indictes. OOPS that did not fix it (it is a bad joint), now you have to dig deeper, so you dig out your oscilloscope and start troubleshooting, while the cement guys stand over you "WHEN is it fixed, it cost us a million rupees (or whatever) standing still". Oh by the way, if you use sockets for equipment in a cement plant, you will have a bunch of trouble after a year or two.

So, you have a) created a troubleshooting environment that is not conductive to thinking and b) you hold up the customer for far longer than a simple replacing the unit would take.

My field guys do not have a single component, they have a case of refurbished boards and our callback rate is about 1%. Many years ago i worked as a fill in after a service manager quit till another could be found and this was a component replacement field service. The callback rate was about 40%.

If the above does not get you away from the component replacement in the field ides, then - good luck.

Erik