DEF Quality Poor — Contamination or Dilution Detected
The VOLVO-MACK-MID128-PID3364-FMI18 (Volvo/Mack D/MP Series) diesel fault code means: DEF Quality Poor — Contamination or Dilution Detected. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- advanced
- Estimated cost
- $500-$2,500
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Common Symptoms
- DEF warning lamp illuminated
- Countdown to engine derate (usually 50 hours)
- Low DEF or poor DEF quality messages
- SCR efficiency low (NOx emissions up)
- 5 mph limp mode if ignored past countdown
Probable Causes (Ranked by Likelihood)
- DEF contaminated with water, fuel, or incorrect fluid Very Likely
- DEF diluted by rain or wash-water entry through cap Likely
- Failed DEF quality sensor Possible
- Old DEF degraded from UV or heat (over 2 years) Possible
- ACM calibration or software fault Less Likely
Step-by-Step Diagnostic Procedure
Connect Volvo PTT or Mack Guided Diagnostics and read DEF quality reading and fault history.
Test DEF with a refractometer — must read exactly 32.5% urea concentration; anything off indicates contamination.
Drain and replace DEF with fresh ISO 22241 compliant fluid; inspect tank for water, rust, or sediment.
Inspect DEF cap seal and tank vent for water intrusion points.
After DEF replacement, clear the fault and re-run SCR self-test via PTT or Guided Diagnostics.
If the fault returns with good DEF, replace the DEF quality sensor.
Frequently Asked Questions
What does Volvo/Mack code MID128-PID3364-FMI18 mean?
Volvo/Mack code MID128-PID3364-FMI18 (DEF Quality Poor — Contamination or Dilution Detected) is a serious-severity diagnostic trouble code set on D11/MP7, D13/MP8, and D16/MP10 diesel engines in Volvo VNL/VNR and Mack Anthem/Pinnacle/Granite/Titan trucks. It indicates a fault in the DEF quality system and should be read with Volvo Premium Tech Tool (PTT) or Mack Guided Diagnostics.
Can I drive a Volvo or Mack truck with code MID128-PID3364-FMI18?
Short distances may be acceptable to reach a repair facility, but the fault will escalate to derate (typically 55 mph then 5 mph on aftertreatment faults). Do not defer diagnosis beyond a shift.
How much does it cost to fix Volvo/Mack code MID128-PID3364-FMI18?
Typical repair falls in the range $500-$2,500. Sensor/wiring repairs are at the low end; EGR cooler, NOx sensor set, DEF pump, injector, or head work runs at the high end. Volvo PTT or Mack Guided Diagnostics is normally required for calibration and parameter programming.
Sources
This page is built from documented references. Verify against your own service info before repair work.