Truck Fault Codes

International A26 SPN 3361 FMI 1 — DEF Supply Pressure Below Normal

SPN 3361 FMI 1 on an International A26 means DEF supply pressure is below the acceptable range. Check the DEF filter, supply lines, and pump output. International ServiceMaxx can monitor live DEF supply pressure and test the pump to distinguish a restriction from a pump failure.

Code Details

Structured details for SPN 3361 FMI 1
Display codeSPN 3361 FMI 1
SPN3361
FMI1
OEM codeNone listed
ManufacturerInternational
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 International A26 DEF system uses a pump to maintain the pressure needed to deliver DEF to the dosing injector. When supply pressure drops below the calibrated minimum, the SCR system cannot dose accurately and NOx conversion suffers. On A26-powered LT and RH Series trucks, a clogged DEF filter, a supply line problem, or a degraded pump are the typical causes. International ServiceMaxx is the appropriate tool for diagnosing and verifying the repair.

SPN 3361 is related to Aftertreatment DEF Dosing Unit Pressure in J1939. FMI 1 indicates a below-normal pressure reading. On International A26 engines, the DEF supply system is monitored by the aftertreatment control module. When supply pressure falls below the calibrated threshold, SPN 3361 FMI 1 is logged and the inducement sequence may begin if the fault is not resolved. International ServiceMaxx provides live DEF supply pressure data and supports pump activation testing.

Common Symptoms

  • DEF system or aftertreatment warning on the International instrument cluster
  • SCR efficiency or NOx conversion fault alongside the pressure code
  • DEF dosing quantity below commanded levels in ServiceMaxx
  • Potential cold-weather occurrence before the DEF heating system completes warm-up
  • Inducement sequence progression if supply pressure fault is not corrected

Possible Causes

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

  • Plugged DEF filter — first component to check
  • DEF supply line blocked, kinked, or frozen
  • DEF pump motor degraded or failing
  • DEF pressure sensor returning a low reading
  • Contaminated DEF fouling the filter and pump

First Checks

  • Replace the DEF filter as the first service action when diagnosing supply pressure faults
  • Allow the DEF heater to complete warm-up in cold weather before pursuing additional diagnosis
  • Connect International ServiceMaxx and monitor DEF supply pressure during a pump activation cycle
  • Inspect the DEF supply line routing for kinks or damage
  • Verify DEF quality meets ISO 22241 and that the tank level is adequate
  • If the filter and lines are clear and pressure remains low, test the DEF supply module for pump output capacity

Can I Keep Driving?

The A26 can operate initially with SPN 3361 FMI 1 active, but SCR dosing will be insufficient and the inducement sequence can begin. Repair the pressure fault to prevent inducement escalation.

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

Related Lookup Pages

Sources

  • International Trucks Service Support Public Resources International Trucks / Navistar International · oem · accessed 2026-06-11 · confidence medium

    Source: International Trucks / Navistar International, International 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 DEF supply module on the International A26 field-serviceable?

The DEF filter within the A26 DEF module is typically a replaceable cartridge. The pump assembly may be replaceable as a subcomponent or may require replacing the entire module depending on the design and failure mode. International ServiceMaxx diagnostic data showing the pressure output before and after filter replacement helps determine whether the pump itself needs attention.

Can a soft or collapsing DEF supply line cause SPN 3361 FMI 1 on the A26?

Yes. A DEF supply line that collapses under negative pressure when the pump draws fluid — due to age, heat exposure, or a fitting that has partially separated — can restrict flow and reduce supply pressure even though the line appears intact when the pump is off. Inspecting DEF lines for soft spots, collapse marks, or partial separations at fittings is worthwhile when filter and pump tests are inconclusive.

What happens to the A26 SCR system if SPN 3361 FMI 1 is ignored for an extended period?

If supply pressure remains inadequate, DEF delivery to the dosing injector will be unreliable or absent, and the SCR catalyst will not reduce NOx to acceptable levels. The aftertreatment control module will log SCR efficiency faults and start the operator inducement sequence, which escalates through progressive torque and speed restrictions over accumulated mileage. Prolonged operation without adequate DEF supply can also cause dosing injector deposits from incomplete urea injection events.