I can hazard a guess.

Are those kits made for gas cars?

Maybe they’re looking for CO or some combustion product that you don’t find in diesel exhaust.

Just a guess.

Try holding the solution in front of a gas car’s pipe. Maybe that won’t work either post cat? I guess it would be best to call the manufacturer and see what they’re looking for.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles