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Old 06-03-2023, 09:14 AM   #1
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How many miles will DEF last

We filled up the DEF tank 2 days ago and drove 800 miles. We are about to leave and tank is showing empty. Can anyone tell me what is going on?
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Old 06-03-2023, 09:28 AM   #2
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DEF mileage depends on engine fuel mileage. If your coach averages around 8.0 MPG fuel mileage you should get around 400 miles on a gallon of DEF. So if you filled the DEF tank and drove 800 miles you should have used approximately 2.0 gallons of DEF. If your tank is reading empty and it's the normal 9.0 to 13.0 gallon size, your tank level sensor is malfunctioning.
You should have two tank level gauges, one on the tank itself and one on the dash in the fuel gauge.

My tank gauge quit years ago, I just rely on the dash gauge now.
2013 Tiffin Phaeton 40QBH
2016 Chevrolet Equinox LTZ AWD V6
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Old 06-03-2023, 09:54 AM   #3
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It should not be empty after only 800 miles and as noted you should have only used 2-3 gallons. What is the age of your MH? One of the symptoms of a failing DEF head is showing empty when it's not. Are you seeing any malfunction lights or trouble codes?
2017 Phaeton 40IH XSH Maroon Coral - Power Glide Chassis with IFS
Previous '15 Tiffin Allegro RED 38QRA and '06 Itasca Sunrise 35A
'16 Jeep JKU Wrangler Sahara or '08 Honda Goldwing
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Old 06-03-2023, 10:16 AM   #4
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Originally Posted by Maisa View Post
We filled up the DEF tank 2 days ago and drove 800 miles. We are about to leave and tank is showing empty. Can anyone tell me what is going on?
Depending on year model and make, some def tank gauges on the tank need to have the ignition key on. You may know this, but others may not. Our tank has a notice label attached that says this.
Eddie and Jomaye, Retired
2018 Newmar Ventana 4369
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Old 06-03-2023, 10:58 AM   #5
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You'll burn between 2% - 3% of the total fuel used in DEF. Easy and safe math is to just figure 3% of fuel used. If you used 100 gallons of fuel, you should have used 3 gallons of DEF.

Are you actually using the amount of DEF on your gauge.....have you added DEF and noted how much it took. It sounds like a gauge issue or as others have stated, a DEF head failure. The DEF head failures typically throw three different codes. The most problematic DEF heads have been on the Spartan chassis and some of the Tiffin Powerglide chassis.
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