Never experienced it myself but i read that if it runs out of DEF or thinks that it has run out, the car will not start because running it w/o the DEF would pollute the environment so they programmed it to not start w/o DEF. That could be a headache if it senses no DEF on the tanks :x

I just sent some info to EPA about this issue, hopefully someone looks into it.

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2011 35d