Medium urgencyMay trigger reduced-power derateaftertreatment

SPN 4364 FMI 18: DEF quality or level – abnormal

Aftertreatment DEF (diesel exhaust fluid) quality or level fault. May trigger derate or warning.

Aftertreatment faults often affect regen behavior, DEF quality messages, and engine derate events.

Driveability

Reduced power or derate is possible, so capture the code context before the condition worsens.

Module context

This code is currently grouped under the ACM so the assistant can frame likely causes and related systems faster.

Best next step

Capture the full fault list and operating condition before clearing anything. That gives the shop or roadside technician a much better starting point.

What symptoms often appear with SPN 4364 FMI 18

  • Check engine light with reduced power or derate
  • DEF, SCR, or regen messages on the dash
  • Recent quality, dosing, or exhaust aftertreatment alerts

Low-risk checks before shop-level diagnostics

  • Record the full fault list and note whether the truck is already in derate.
  • Check DEF level and confirm no incorrect fluid was added.
  • Look for obvious leaks, smoke, or heat concerns before continuing.

Related procedures and shop prep

Check DEF level and quality

driver

Verify DEF tank level and condition; top off with certified DEF if low.

Related reading

More aftertreatment fault-code pages

Back to all fault codes

SPN 3710 FMI 2

DPF differential pressure – erratic or intermittent

DPF (diesel particulate filter) pressure sensor or circuit fault; can affect regen and derate.

View related code

FAQ

Frequently asked questions about SPN 4364 FMI 18

What does SPN 4364 FMI 18 usually mean on a truck?

SPN 4364 FMI 18 commonly points to def quality or level – abnormal. In TruckHelpNow, it is grouped under the aftertreatment subsystem so drivers and shops can start with the most relevant checks first.

Can I keep driving with SPN 4364 FMI 18?

Reduced power or derate is possible, so capture the code context before the condition worsens. The final decision should still depend on the full warning picture, related symptoms, and whether the truck is clearly safe to continue moving.

What should I collect before calling a shop about SPN 4364 FMI 18?

Collect the full code list, when the issue appears, what changed first, and any related warning lights or recent repairs. That context helps the next diagnostic step stay focused.