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Hi my name is***** many miles to the gallon are you getting? It does not sound unreasonable what you are describing. Is it reading empty when it's not? I want to be sure I understand what you are asking. Let me know
Is this reading empty when its not?
Or are you just not happy about the tanks capacity?
It is normal to get a low DEF warning light when the system is low. Let me know
This is right from the manual. click the link below ans see manual pages 143 on and then 178. Let me know if this helps answer your questions.
If the DEF reserve has fallen below the reserve range, add at least 2.0 US gal(7.6 l) of DEF at the latest after receiving the second warning message
It explains how the level indicator reads and resets after a fill or top off.