If I understand your post, the inquiry goes to whether code readers are useful on pre-def engines.
The OBD-II standard that code readers work with was already implemented on vehicles we bought in 2008. A large number of viewable parameters and possible fault codes can be looked at using a code reader.
Our 2007 Ram 2500 pre-def diesel truck and our current 2024 Ram 2500 def-equipped diesel truck have both seen great benefit from having a code reader tool.
Opinions vary on type and brand of code readers people prefer. Some are stand alone, and some use a bluetooth connected plug-on interface with phone app.
Not all vehicles permit simple code resetting. We had to buy a "Gateway Bypass Cable" add-on for the 2024 Ram.
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