2018 Freightliner Cascadia - DD15 SPN 3364 FMI 2 - DEF Quality Low Derate

Question

My truck just went into a 5 MPH derate while pulling a heavy load through the mountains. The dash is screaming about poor DEF quality even though I just filled up at a major truck stop. I need to get this moving ASAP because I have a tight delivery window.

obd2_owl · Accepted Answer

The issue was a failed DEF Quality Sensor integrated into the tank header assembly. I used a refractometer and confirmed the DEF was exactly 32.5%, but the sensor was reading 41%. I replaced the DEF Header (Part # A06-90577-002) which cost about $650, performed a parked regen, and used Diagnostic Link to reset the SCR NOx Efficiency and DEF Quality learned adaptives. The derate cleared immediately and the truck is back to full power.

fleet_chief_42 · Answer

If you don't have a refractometer, take a small sample of the DEF in a clear water bottle. It should be crystal clear; if it looks cloudy or smells like ammonia, your fluid is definitely contaminated with diesel or oil.

Dave Kowalski · Answer

Check the DEF tank header coolant valve. Sometimes if that valve sticks open, it cooks the DEF fluid and throws the concentration sensor off, making the ACM think the fluid is bad.

backyard_bart · Answer

You need to perform a 'DEF Quantity Test' and a 'SCR Efficiency Test' using Detroit Diagnostic Link. Even if the fluid is good, the sensor itself might have crystallized buildup on the sensing rods.

2018 Freightliner Cascadia - DD15 SPN 3364 FMI 2 - DEF Quality Low Derate

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