Truck Fault Codes

GM Duramax L5P SPN 1569 FMI 31 �?Engine Protection Derate Active

SPN 1569 FMI 31 on a GM Duramax L5P means the engine protection derate is active. Use a GM-compatible scan tool (Tech 2, GDS2, or an advanced OBD2 reader) to identify the companion DTCs �?DEF empty or quality failure, SCR efficiency issues, high coolant temperature, or oil pressure faults are common triggers.

Code Details

Structured details for SPN 1569 FMI 31
Display codeSPN 1569 FMI 31
SPN1569
FMI31
OEM codeNone listed
ManufacturerGM
SystemEngine protection / aftertreatment
ComponentECM engine protection strategy
Source addressUnknown or not applicable
Severityhigh
Review statussource backed
Source confidencemedium
Last reviewed2026-06-11

Plain-English Meaning

The L5P's ECM monitors all critical engine and aftertreatment parameters. When a protected threshold is crossed, the ECM reduces engine power and torque to protect the engine or maintain emissions compliance. The SPN 1569 FMI 31 code confirms a derate is in effect �?the companion codes tell you exactly what is causing it. On GM Sierra 4500/5500 and Express 4500 commercial trucks with the L5P Duramax, the SCR inducement system follows the same EPA-mandated escalation structure as other compliant platforms. GDS2 displays the active fault codes feeding the inducement, the counter status, and the aftertreatment system health metrics. For fleet L5P trucks that cover low annual mileage but make many short trips, maintaining fresh DEF in the tank is the most important preventive measure — DEF that sits in the tank for extended periods in warm weather can degrade and trigger quality faults.

SPN 1569 FMI 31 is the Engine Protection Torque Derate active indicator on J1939. On the GM Duramax L5P (RPO code L5P, introduced for 2017 model year), the ECM monitors DEF level, DEF quality, SCR efficiency, DPF loading, coolant temperature, and oil pressure. A derate from SCR inducement follows a prescribed escalation: warning at low DEF �?reduced speed at empty �?5 mph max if DEF quality has been ignored for an extended period.

Common Symptoms

  • Reduced engine power and torque output
  • Wrench light or reduced engine power message on the gauge cluster
  • DEF or emission system warning messages
  • Companion DTCs visible in a scan tool

Possible Causes

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

  • DEF tank depleted �?the most common cause of SCR-triggered derate
  • DEF quality below acceptable concentration
  • SCR catalyst degraded or dosing injector plugged
  • High coolant temperature from a cooling system fault
  • Low engine oil pressure
  • DPF critical soot loading from insufficient regen

First Checks

  • Check DEF tank level �?top off with GM-approved DEF if low
  • Read all active DTCs with a GM-capable scan tool
  • Check coolant level and engine oil level
  • Do not clear codes without resolving the underlying trigger
  • Connect GDS2 and identify all active fault codes contributing to the L5P SPN 1569 inducement
  • For vehicles with low annual mileage, replace the DEF with fresh fluid before resetting to rule out degraded DEF as the cause

Can I Keep Driving?

The L5P is in a protection derate state. If the companion code indicates an aftertreatment issue, the vehicle is driveable with reduced power. If the trigger is a cooling or oil system fault, stop and assess before continuing.

SPN 3364 FMI 1Aftertreatment �?DEF system · DEF quality / concentration sensorSPN 5246 FMI 31Aftertreatment �?SCR system · SCR catalyst / NOx sensors

Related Lookup Pages

Sources

  • GM Trucks Service Support — Duramax Diesel Resources General Motors LLC · oem · accessed 2026-06-11 · confidence medium

    Source: General Motors LLC, GM Trucks Service Support — Duramax Diesel 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

How many miles can I drive a Duramax L5P in derate mode before it enters 5 mph limp mode?

The progressive inducement strategy for the L5P depends on what triggered the derate. For DEF depletion, the escalation from power reduction to 5 mph limit typically follows a distance-based countdown. The exact countdown depends on the GM calibration version. Refilling the DEF tank at any point in the escalation sequence will clear the countdown and restore power, assuming no other faults are present.

Can I tow with a GM Duramax L5P that has SPN 1569 FMI 31 active?

Towing under a derate condition is not recommended. The reduced torque output may be insufficient for the load, creating a dangerous situation, particularly on grades. Diagnose and resolve the derate trigger before resuming towing operations.

Will clearing the DTC with a scan tool resolve SPN 1569 FMI 31 on the Duramax L5P?

Clearing the code without fixing the root cause is a temporary measure �?the code will return as soon as the triggering condition is detected again, which may be immediate. Address the companion code root cause first, then clear codes and verify the fault does not return over a drive cycle.