DEF Quality Poor — Contamination or Dilution Detected
What does VOLVO-MACK-MID128-PID3364-FMI18 mean?
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.
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.