Def

Need more info, what engine/truck/year/model? how many miles?

That said considering my own trucks massive list of problems it could be #### near anything. Cheapest parts to rule out are the quality sensor, the doser and the NoX sensors. From there it gets...pricey.

 

You said the gauge isn't reading, is the DEF tank actually empty?

If the DEF tank module isn't reading properly on some trucks the gauge needle can point straight down or it'll have a light that will illuminate or flash.

 

Likely a borked quality sensor and/or module in the tank. They seem to die around 55k or so in my experince. Try that first.

 

If it was at half and then suddenly dropped to Empty, I wonder if the sensor got hung up at half a tank, then freed up and dropped once the tank was nearing empty? Cheapest and quickest thing to try if that's the case would be to put a couple gallons of DEF in the tank and see if that does anything.