DEF (diesel exhaust fluid) is an important component of the emission control system on modern diesel engines. It helps to reduce harmful emissions, such as nitrogen oxides, by breaking them down into harmless nitrogen and water. If the DEF level gauge readings are inaccurate or erratic on your Peterbilt 389, 579 or other model with a Cummins ISX engine, it can cause problems with the emission control system and may result in failed regenerations, excessive or not enough DEF consumption, 5mph derate, DEF crystallization build up in the SCR system etc.
Here are some common causes of inaccurate or erratic DEF level gauge readings on your Peterbilt semi truck with a Cummins ISX engine:
- Actually Low DEF fluid level: This may seem obvious, but if you’re near empty on your tank it can do weird things. This is the most common cause of inaccurate or erratic DEF level gauge readings. If the DEF fluid level is low, the gauge or dipstick may not accurately reflect the actual DEF level.
- Faulty DEF level sensor aka DEF sending unit: If the DEF level sensor or wiring is not functioning properly, it may not accurately detect the DEF fluid level and the vehicle's computer system may not display the correct DEF level on the dashboard gauge or dipstick. This can cause the DEF level to fluctuate or appear to be erratic. The DEF level sensor is part of a multi-purpose component that also includes the DEF quality sensor, DEF tank temperature sensor, and DEF tank heater. Please note if you are seeing DEF fault codes for any of these components you may have a malfunctioning DEF sending unit aka header, which is mounted on top of the DEF tank and looks like a cap with a clamp, but has tubes and wiring that goes down into the tank.
The following causes pertain more toward that consumption or lack of consumption of DEF, but in some cases can cause incorrect DEF level readings:
- Leaks in the DEF system: If there is a leak in the DEF tank, pump, lines, or injectors, the DEF fluid level may be low and the gauge or dipstick may not accurately reflect the actual DEF level.
- Faulty DEF dosing system: If the DEF dosing system is not functioning properly, it may not inject the proper amount of DEF fluid into the exhaust stream. This can cause the DEF to be consumed at an incorrect rate.
- DEF quality issue: If the DEF fluid is of poor quality or has become contaminated, it may not work properly in the emission control system. This can cause the DEF to be consumed at an inaccurate rate and/or the level to appear low, even if the DEF tank is full.
- DEF filter: If the DEF filter is plugged up or dirty it can cause inaccurate readings due to higher or incorrect pressures and flow rates. The DEF filter is located on the bottom of the DEF pump and is inside of a filter cap.
- Exhaust system or other issue: If there is a problem with the exhaust system, such as the aftertreatment fuel system, turbocharger or VGT actuator, EGR valve or a sensor on the engine it may interfere with the proper functioning of the DEF system.
To diagnose and fix the cause of inaccurate or erratic DEF level gauge readings on your Peterbilt big rig with a Cummins ISX engine, you can follow these steps:
- Check the DEF level using the gauge or dipstick. If it is low, add DEF fluid as needed to bring it to the proper level.
- The DEF level sensor can also be a cause of inaccurate or erratic DEF level gauge readings. If the DEF level sensor or wiring is not functioning properly, it may not accurately detect the DEF fluid level and the vehicle's computer system may not display the correct DEF level. To check if the DEF level sensor is the cause of the issue, disconnect the DEF level sensor from the vehicle's computer system and check the DEF level using the gauge or dipstick. If the DEF level is at the proper level, the DEF level sensor may be the issue. Inspect the DEF level sensor for any signs of damage or malfunction, such as cracks, corrosion, or loose connections. If the DEF level sensor is found to be faulty, replace it with a new one.
- If the DEF level is still low or the gauge readings are still inaccurate after replacing the DEF level sensor, there may be another issue causing the problem. Have the DEF fluid and exhaust system inspected by a professional to determine the cause of the issue.
If the DEF level is already at the proper level, the gauge shows the accurate readings, but if the DEF consumption is abnormal, the issue may be a leak in the DEF system. Inspect the DEF tank, pump, lines, and injectors for any signs of leakage.
- If a leak is found, repair or replace the faulty component as needed.
- If no leak is found and the DEF consumption is still incorrect, the issue may be with the DEF dosing valve aka DEF doser or injector. This dosing valve is responsible for injecting the DEF fluid into the exhaust stream. In order to diagnose the DEF injector dosage amount, a diagnostics scan tool capable of performing DEF activation tests will be required. You can also remove the injector and take a look inside of the DPF assembly base to check for DEF crystallization build up. A more accessible way would be to remove the exhaust pipe that the DEF injector mounts to and take a look inside through the flange. IF build up is found it’s a pretty solid sign that you may have a faulty DEF doser. If the DEF injector is found to be faulty, repair or replace it as needed.
- You can also take a look at the DEF filter, the DEF filter is located on the bottom of the DEF pump and is inside of a filter cap. Replacing it is not a difficult task, but once removed it should be visually inspected for debris, discoloration etc. and replaced with a new one.
If the DEF level is still low or consuming an incorrect amount of DEF after checking for leaks and inspecting the dosing system, the issue may be caused by components before the DEF system, such as the aftertreatment fuel system, turbocharger or VGT actuator, EGR valve or a sensor on the engine. Have these systems inspected by a professional to determine the cause of the issue.
Once the cause of the inaccurate or erratic DEF level gauge readings has been determined, repair or replace any faulty components as needed to fix the issue. Also remember to replace and gaskets or wear and tear parts when troubleshooting.
