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The closest Vauxhall dealership to my residence is more than 50 miles distant. Two distinct local garages have examined the issue, identified the problem, although it consistently reoccurs, and they fail to ascertain the underlying reason of Code 84, which, as you said, indicates less power in limp mode.

Did any of the garages verify the detection of "powertrain fault codes" beginning with the letter P?
Do your garage bills include any further information indicating a lasting resolution? Do you have information on any warnings associated with the flashing of Code 84 on the dashboard warning lights? The vehicle owner's handbook contains descriptions of warning lights.

This discussion is regrettably becoming circular. Jasta's post number 10 corroborates my post number 4. Code 84 is just a 'Warning'. The use of diagnostic equipment proficient in interpreting GM codes will identify the underlying reason. Upon the identification of these 'hidden codes,' corrective measures may be implemented.