DEF Doser/Injector Circuit Shorted
The VOLVO-MACK-MID128-PID3361-FMI6 (Volvo/Mack D/MP Series) diesel fault code means: DEF Doser/Injector Circuit Shorted. This is a serious severity code.
- Keep driving?
- Short distances only
- 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)
- Failed DEF pump, doser, heater, or sensor in DEF supply module Likely
- Harness damage at DEF tank module (common from road debris) Likely
- DEF line ice blockage (if tank heater failed) Possible
- Contaminated DEF causing pump or doser fouling Possible
- ACM (Aftertreatment Control Module) fault Less Likely
Frequently Asked Questions
What does Volvo/Mack code MID128-PID3361-FMI6 mean?
Volvo/Mack code MID128-PID3361-FMI6 (DEF Doser/Injector Circuit Shorted) 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 doser 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-PID3361-FMI6?
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-PID3361-FMI6?
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.
What actually makes the computer set this NOx or SCR code?
California's heavy-duty OBD regulation requires the system to flag a NOx-converting catalyst when its conversion capability drops enough to push NOx emissions more than 0.4 g/bhp-hr above the applicable standard (Title 13 CCR 1971.1, section (e)(6.2.1)) — so on a 0.2 g/bhp-hr engine the trip point is around 0.6. Catalysts in series may be monitored individually or as a group, which is why a code can point at the aftertreatment as a whole rather than at one brick. This threshold comes from the heavy-duty OBD regulation, which covers 2010 and later heavy-duty engines. If your engine is in a pickup at or under 14,000 lb GVWR it is certified under the separate medium-duty rule instead, so treat this as the design intent rather than your engine's exact limit.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
Connect Volvo PTT or Mack Guided Diagnostics, verify MID128-PID3361-FMI6 active, and observe DEF pump and doser live data.
Command DEF pump pressure test; pressure should reach 70-130 psi in a few seconds.
Inspect DEF lines for kinks, leaks, or frozen/clogged sections.
Measure heater element resistance at DEF tank module; replace if open.
Check DEF quality with a refractometer; replace contaminated DEF.
After repair, run the SCR initialization and purge sequence via PTT or Guided Diagnostics.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- California Code of Regulations, Title 13 section 1971.1 (Heavy-Duty OBD)malfunction criteria, section (e)
- SAE J1939-73Application Layer: Diagnostics (defines the SPN/FMI fault-identifier framework)
Commonly Affected Vehicles
- Volvo VNL 760/860 with D13
- Volvo VNR 640 with D11/D13
- Mack Anthem with MP8
- Mack Pinnacle with MP7/MP8
- Mack Granite with MP7/MP8
- Mack Titan with MP10
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