Incorrect DEF Quality
The Diesel Exhaust Fluid (DEF) quality is below acceptable levels.
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What it means (plain English)
The computer checks the quality of the Diesel Exhaust Fluid (DEF) used to reduce emissions. If the DEF is contaminated or of poor quality, it will throw this code.
What the computer is actually seeing
The system monitors the urea concentration in DEF. An acceptable level is usually between 32.5% to 34.5% urea. When the concentration dips below this range or becomes contaminated, the sensor detects it and triggers the code.
What a healthy reading looks like
DEF must contain 32.5% urea concentration. Sensors often operate between 0 to 5 volts with ideal outputs around 2.5 volts for quality DEF.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Inspect the DEF quality sensor connector and wiring for any damage or corrosion. If found, repair or replace as necessary.
- 22. Check the DEF fluid for contamination or incorrect type. Drain and replace with known good DEF if contamination is present.
- 33. Use a scan tool to check the voltage output of the DEF quality sensor. It should be around 2.5 volts for good DEF. If not, replace the sensor.
- 44. Inspect the DEF dosing module for proper operation and leaks. Ensure it’s dosing correctly according to ECU commands.
Common causes
- Contaminated DEF
- Wrong type of fluid used (e.g., water or non-urea fluid)
- Failed DEF quality sensor
- Defective DEF dosing module
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with SPN 4446 active?
While the vehicle may still run, it's recommended to address the issue soon to avoid further emissions-related problems or failure to pass inspections.
Will this code clear after replacing the DEF?
Yes, the code should clear once the quality of DEF is verified and the system recalibrates after repairs.
Why does this code appear with other SPN codes?
SPN 4446 is often paired with other DEF-related issues indicating a broader problem with the emissions control system.
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