def= diesel exhaust fluid. Read your manual- is essentially a tank of urea that is used sparingly (and automatically) to reduce the Nox emissions from the diesel. Pay attention to the pitfalls- ignoring this and the vehicle warnings can leave you stranded. Again- Read the Manual.

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Dave

'05 325xi
'10 E70 35d
'85 911 Carrera
'66 911 Work in progress
'91 Chevy C1500
'95 Jeep Cherokee
‘18 Jeep JLU