How can a mechanic claim I have original plugs/wiring harness? on 1996 Lexus SC400

Today an independent ASE certified mechanic told me that
I've never had my spark plugs or wiring harness changed.
My 15 year old Lexus drives like a dream at 122K and has had
regular maintenance at the dealer.

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3 answers
theres a date on your wires possibly
SOMETIMES OE PARTS ARE MARKED WITH A SLASH OF COLORED PAINT
Sometimes there are tell tale signs that parts have never been replaced, but sometimes people can make mistakes.

I would recommend consulting your dealer to obtain full maintenance records - if possible. And/or check your own records to see what the history of work has been. This will make you an educated owner, if nothing else.