SPN 5246 FMI 31: DEF Quality - Poor Quality Detected

Overview\n\nSPN 5246 FMI 31 indicates the ECM has detected poor quality DEF (Diesel Exhaust Fluid). This can lead to reduced NOx conversion and eventual derate.\n\n## Why This Happens\n\nThe DEF quality sensor monitors fluid concentration and properties. When DEF falls outside specifications (too weak or too strong), this code sets.\n\n## Step-by-Step Diagnosis\n\n1. **Check DEF age and source** - Old DEF or off-brand fluid often causes this\n2. **Test DEF concentration** - Use a refractometer (should be 32.5% urea)\n3. **Inspect for contamination** - Look for crystals, debris, or discoloration\n4. **Clear and test** - Replace fluid, clear codes, and monitor\n\n## Prevention\n\n- Always use fresh DEF from reputable sources\n- Check expiration dates before purchase\n- Keep DEF tanks clean and sealed