Duramax diesel exhaust fluid fuel tanks are in vehicles equipped with diesel engines. Selective Catalytic Reduction (SCR) technology uses the DEF and catalytic converter to reduce the overall nitrogen emissions in the environment. DEF tank is usually mounted beneath the trucks and contains deionized water to lower exhaust emissions.
Duramax DEF tank full but says empty can be due to faulty DEF level sensors, bad ECU, damaged reluctant heater, incorrect wiring of sensors, DEF contamination, faulty DEF pump, and air bubbles in the system.
I also faced this issue while driving my truck in California. I saw the warning message on the cluster screen and had the spare fluid bottle in the cargo area. I checked the sensor, but it was fine, and it was also designed to last for 120k miles. I added the fluid to the DEF tank and turned off the ignition to reset the system and remove the errors. I went for a test drive, and the error message disappeared after some miles.
| Causes | Solutions |
| Faulty DEF level sensor | Ensure proper storage conditions |
| Bad ECUs | Reset via a battery of fuse |
| Damaged reductant heater | Replace reductant heater |
| Incorrect wiring of sensor | Rewire the damaged cables |
| DEF contamination | Prevent direct sun and heat exposure |
| Faulty DEF pump | Replace pump |
| Calibration issues | Update software or reprogramming |
| Air bubbles in the system | Bleed the DEF system |
| Use of an incorrect type of DEF fluid | Use certified and compatible DEF |
Faulty DEF level sensor
DEF sensor is the part of the sending unit that measures the level of liquid left in the tank. It is the urea quality level sensor mounted on the upper side of the tank and is also known as the combined unit tank.
The primary function of this device is to monitor the level of liquid remaining in the tank and send the information to your vehicle’s electronic control unit.
The ECU, in return, triggers the warning message via gauges and panels. Sometimes, the sensor becomes faulty and cannot get the correct information to the ECU, leading to misconception and interrupted driving.
These sensors give a false reading, and you can see a warning message even when the tank is full. The sensors become faulty because of contaminated DEF tanks.
The sludge from the contaminated fluid accumulates on this device and causes it to malfunction. In addition, it can also become bad because of storage in a vulnerable environment. It is highly temperature-sensitive and needs to be stored in a dry and cool place.
It cannot function when you park your truck outside in extremely cold and hot weather. You can also face problems when you use non-certified sensors that are not compatible with your vehicle and show incorrect readings.
It is necessary for you to purchase certified and compatible DEF sensors for your trucks. Park your truck in indoor parking areas during cold and hot climatic conditions. Regularly inspect the diesel exhaust fluid tank for signs of contamination, cracks, and leakages.
Bad ECUs
DEF level sensors send the information to the electronic control unit. These small devices are mounted in your vehicle’s body and control specific functions.
Each of these units is specific to its corresponding component and only functions when you operate the respective part. The ECU unit can also malfunction and give incorrect readings on the display screen.
It is challenging for drivers to receive a message that the tank is empty even if they filled it a few miles before. You can reset the electronic control module to restart its functioning.
You can reset it manually by disconnecting the battery and removing the ECU fuse from the box. Locate the fuse box, remove the fuse, and wait for 10 to 15 minutes.
Damaged reductant heater
The reductant heater is part of the diesel exhaust fluid system that reduces the crystallization and freezing of the fluid. My friend also faced the same issue and said he found the diagnostic trouble code P20BD on the screen.
The specific DTC shows the malfunctioning of the reductant heater. It is located in the filter reservoir or injection hose supply of the exhaust fluid system, depending on the model of the vehicle.
You can receive the DEF tank empty message on the screen because of the failure of this heater. Failure of the heater causes freezing of the fluid, and it cannot remove the nitrogen exhaust emissions.
The system detects that the tank is not full, indirectly increasing emissions. Reductant heaters become faulty because of the temperature sensor and glow plug module malfunctioning.
You can check the malfunctioning of the reductant heater from the diagnostic code. You can fix the P20BD diagnostic trouble code by replacing the reductant heater with a new one to restore the engine performance and clear the check engine light.
Incorrect wiring of sensor
Sensors send the signals to the ECU via the channel of wires, triggering the warning sign on the screen. Level sensors are connected to the display screen through these wires.
You can see inaccurate readings on display because of damaged and corroded electric cables. Broken wires show the tank is empty when it is full of fluid. It happens because of the incorrect signaling of the sensors.
Track the wires that run from the level sensors to the ECU. Replace the damaged wires with new ones to ensure the correct signaling.
DEF contamination
DEF contamination can make the sensors dirty, showing the incorrect reading on the cluster screen. Your fluid tank is empty, but the error code indicates it is empty because of the contaminated sensors.
It gets contaminated because of heat exposure. Heat increases the risk of cross-contamination and causes premature exhaust fluid degradation.
The issue occurs because of the storage in hot climatic conditions. In addition, direct sunlight exposure can also worsen the situation. It is necessary for you to park your trucks in indoor parking lots to protect them against sun and heat exposure in hot weather.
Faulty DEF pump
DEF pump is mounted on the tank, which ensures the fluid supply to the dose injectors, delivering this fluid to the selective catalyst reduction chamber.
Sometimes, this pump becomes bad and cannot supply the fluid to the SCR chamber, and you can see a warning notification on the display screen.
The pump is vulnerable to failure because of electric issues. It can get stuck and does not allow the smooth fluid flow to the SCR chamber. It can also size because of the dirt and metal contamination from the contaminated tank.
It is better to replace the faulty pump with a new one. Keeping the diesel exhaust fluid tank clean can decrease the chances of seizing the pump.
Calibration issues
The engine control module is also vulnerable to calibration issues because of software glitches and programming errors. Non-calibrated ECU leads to the incorrect functioning of the level sensors because of the faulty readings on the screen.
Calibration issues usually come because of the high mileage of the trucks, which causes programming errors. In addition, it also happens when you do not timely consult the dealerships to update the software.
Update the software in your trucks whenever an update notification pops up on the cluster screen. Furthermore, you can also reprogram the system to remove the programming errors.
Air bubbles in the system
Air bubble formation in the Duramax DEF tank also causes inaccurate readings, and your display screen shows a low fluid level when it is normal.
The issue comes because of the air bubble in the tank, which can lead to incorrect readings by the sensors. Air can get inside because of the damaged and leaky vacuum and fluid lines.
Moreover, air can enter this system when you do not flush the fluid tank for a long time. You can remove the air bubbles by bleeding the tank to get rid of trapped air.
Use of an incorrect type of DEF fluid
All diesel exhaust fluids are not the same because of differences in manufacturing and consistencies. You have to select it according to the diesel engine type and assembly.
Many people select the Duramax fluid, but these are not compatible with the fuel lines and hoses, leading to inaccuracies.
Using non-compatible fluid can also affect the sensors, which become faulty. You should purchase the certified and compatible DEF from the hardware store to reduce the chances of error.
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