Truck Fault Codes

Mack MP8 SPN 3361 FMI 1 — DEF Supply Pressure Below Normal

SPN 3361 FMI 1 on a Mack MP8 means DEF supply pressure is below the normal operational range. The DEF supply pump is not maintaining the pressure needed for dosing injector operation. Check the DEF filter, supply lines for blockage, and pump output. Mack Premium Tech Tool (PTT) provides live DEF pressure data for diagnosis.

Code Details

Structured details for SPN 3361 FMI 1
Display codeSPN 3361 FMI 1
SPN3361
FMI1
OEM codeNone listed
ManufacturerMack
SystemAftertreatment — DEF supply system
ComponentDEF supply pump / DEF lines / filter
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-12

Plain-English Meaning

The Mack MP8 uses a DEF supply pump to pressurize DEF from the tank to the dosing injector. When supply pressure is insufficient, DEF delivery to the SCR catalyst is compromised, reducing NOx conversion and risking the inducement sequence. This fault is most commonly caused by a plugged DEF filter, a supply line issue, or a pump that is beginning to fail. Mack PTT is used to confirm the pressure reading and test the pump.

SPN 3361 is related to Aftertreatment DEF Dosing Unit Pressure in J1939. FMI 1 indicates a below-normal pressure reading. On Mack MP8 engines, which share the Volvo Group D13 aftertreatment architecture, the DEF supply module is monitored by the aftertreatment control system. When supply pressure falls below the calibrated minimum, SPN 3361 FMI 1 is logged. Mack PTT can monitor live DEF supply pressure and command the pump for diagnostic testing.

Common Symptoms

  • DEF system or SCR warning on the Mack instrument cluster or GuardDog Connect
  • Possible SCR efficiency fault alongside the pressure code
  • DEF dosing rate below commanded value in PTT data
  • More likely to occur after cold starts in winter before the DEF system has fully warmed
  • Inducement sequence may begin if the fault persists without correction

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • DEF filter plugged — most common first cause to investigate
  • DEF supply line blocked, kinked, or frozen
  • DEF supply pump failing — reduced output pressure
  • DEF pressure sensor fault
  • Air in the DEF supply circuit after maintenance or tank drain

First Checks

  • Replace the DEF filter as a first step when diagnosing low supply pressure, especially if the truck is due for a service interval
  • Allow the DEF heater to complete its warm-up cycle before pursuing diagnosis on a cold-weather first occurrence
  • Connect Mack PTT and monitor live DEF supply pressure during pump activation
  • Inspect the DEF supply line from the tank to the dosing module for kinks or damage
  • Check DEF quality and level — contaminated DEF can foul the filter and pump faster than expected
  • If pressure remains low after filter replacement and line inspection, test the pump module for adequate output

Can I Keep Driving?

The MP8 will initially operate with this fault active, but SCR dosing will be reduced and the inducement sequence can begin. Repair the DEF supply pressure fault before the inducement advances to a torque or speed restriction.

SPN 3480 FMI 5Aftertreatment �?DEF dosing system · DEF dosing injector / wiring circuit

Related Lookup Pages

Sources

  • Mack Trucks Service Support Public Resources Mack Trucks Inc. · oem · accessed 2026-06-11 · confidence medium

    Source: Mack Trucks Inc., Mack Trucks Service Support Public Resources. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium

    Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source

FAQ

Is the Mack MP8 DEF supply module the same as the Volvo D13 module?

The Mack MP8 and Volvo D13 share the same Volvo Group aftertreatment architecture, and the DEF supply modules are similar in design and service requirements. Part numbers may differ by specific vehicle application and production year. Service procedures for filter replacement, pump testing, and pressure diagnosis are similar for both engines, though each brand uses its own OEM tool — PTT for Mack and VCADS Pro or Tech Tool for Volvo.

Can air in the DEF supply system on the Mack MP8 cause SPN 3361 FMI 1?

Yes. Air introduced into the DEF supply circuit during filter replacement, line disconnection, or tank service can cause low or erratic pump pressure. Many DEF systems require a priming cycle after service to purge air from the supply path. PTT or a manual pump priming procedure per the Mack service documentation should be followed after any maintenance that opens the DEF supply circuit.

Does cold weather increase the frequency of SPN 3361 FMI 1 on the Mack MP8?

Yes. DEF freezes at approximately minus 11 degrees Celsius. In cold weather, the DEF tank, supply lines, and pump module can contain frozen DEF that restricts the supply path until the heating system thaws them. A first-occurrence pressure fault during a cold-weather start that clears as the system warms up may indicate borderline DEF heating performance. Persistent cold-weather pressure faults warrant inspection of the DEF heater circuit.