What does SPN stand for in truck fault codes?

SPN stands for Suspect Parameter Number. It is part of the SAE J1939 fault code system used in heavy-duty trucks. The SPN identifies which specific component, sensor, or parameter has a problem. There are over 10,000 defined SPNs in the J1939 specification.

What does FMI stand for in truck diagnostics?

FMI stands for Failure Mode Identifier. It describes HOW a component has failed. For example, FMI 0 means the value is above normal range, FMI 1 means below normal range, FMI 3 means voltage is above normal, and FMI 4 means voltage is below normal. There are 32 defined FMI values.

How do I clear SPN/FMI fault codes on a truck?

To clear J1939 fault codes, connect a heavy-duty scanner to the 9-pin diagnostic port, navigate to the fault code menu, and select Clear Faults or Reset Codes. The scanner sends a DM3 (Diagnostic Message 3) command on the J1939 bus. Note: some codes cannot be cleared until the underlying condition is repaired, and some derate codes require an engine power cycle after clearing.

Are SPN codes the same across all truck brands?

Yes, SPN numbers are standardized by the SAE J1939 specification and are the same across Cummins, Detroit Diesel, PACCAR, Volvo, Navistar, and other manufacturers. SPN 3251 always means DPF Soot Load regardless of the engine brand. However, some manufacturers also define proprietary SPNs (typically in the 520000+ range) for brand-specific features.

How to Read SPN/FMI Fault Codes on Heavy Duty Trucks

March 8, 2026

TL;DR: J1939 fault codes consist of an SPN (Suspect Parameter Number — identifies the component) and FMI (Failure Mode Identifier — describes the failure type). Connect a heavy-duty scanner to the 9-pin port, read codes, and decode using the SPN/FMI pair. SPNs are standardized across all truck brands.

If you are used to reading OBD-II codes like P0420 on cars, heavy-duty truck fault codes will look foreign at first. Instead of a single alphanumeric code, trucks use a two-part system: an SPN that tells you what is wrong and an FMI that tells you how it failed.

Once you understand the system, it is actually more precise and informative than OBD-II. This guide walks you through reading and interpreting SPN/FMI codes step by step.

Part of our protocol education series. See also: J1939 vs J1708 vs OBD2 — Protocols Explained and What Is SAE J1939?

Step-by-Step: Reading Fault Codes

Step 1: Connect the Scanner

Locate the 9-pin Deutsch diagnostic connector under the driver-side dash. On most trucks, it is near the steering column or kick panel, often with a rubber dust cap. Connect your heavy-duty scanner using the 9-pin cable and turn the ignition to ON.

Step 2: Navigate to Fault Codes

Select your truck manufacturer on the scanner (e.g., Freightliner, Peterbilt, Kenworth), then navigate to Fault Codes or DTC. Select Read Active Codes to pull DM1 messages from all ECUs on the J1939 bus. You can also select Stored/Inactive Codes to see DM2 (previously active) faults.

Step 3: Read the SPN

The SPN (Suspect Parameter Number) is the first number and identifies the specific component or parameter with the issue. SPNs range from single digits to six-digit numbers. Some common SPNs you will see frequently:

SPN 91 — Accelerator Pedal Position
SPN 100 — Engine Oil Pressure
SPN 102 — Boost Pressure (Intake Manifold)
SPN 110 — Engine Coolant Temperature
SPN 175 — Engine Oil Temperature
SPN 3251 — DPF Soot Load Percent
SPN 3936 — SCR Catalyst Conversion Efficiency
SPN 4364 — Aftertreatment SCR Operator Inducement Severity

Step 4: Read the FMI

The FMI (Failure Mode Identifier) is the second number and describes the nature of the failure. There are 32 defined FMIs:

FMI	Meaning	Typical Cause
0	Data Valid But Above Normal Range	Parameter exceeded upper threshold
1	Data Valid But Below Normal Range	Parameter below lower threshold
2	Data Erratic, Intermittent, or Incorrect	Loose connection, wiring issue
3	Voltage Above Normal	Short to power, open ground
4	Voltage Below Normal	Short to ground, open circuit
5	Current Below Normal (Open Circuit)	Broken wire, disconnected sensor
6	Current Above Normal (Short Circuit)	Shorted actuator, wiring short
7	Mechanical System Not Responding	Stuck valve, seized actuator
9	Abnormal Update Rate	CAN bus communication issue
10	Abnormal Rate of Change	Sensor signal changing too fast
12	Bad Intelligent Device or Component	Failed ECU or smart sensor
14	Special Instructions	See manufacturer service manual
16	Moderately Severe — Above Normal	EPA emissions-related severity level
18	Moderately Severe — Below Normal	EPA emissions-related severity level
19	Received Network Data in Error	CAN bus data corruption
31	Condition Exists	General flag that condition is active

Step 5: Look Up the Combined Meaning

The SPN tells you the component; the FMI tells you the failure type. Combined, they give you a precise diagnosis. Your scanner should display this in plain language automatically.

Step 6: Determine Repair Action

Use the decoded fault to guide your repair. Check for related codes from other ECUs — multiple codes often point to a single root cause. After repair, clear codes using your scanner's Clear Faults function and verify the code does not return.

Common SPN/FMI Codes You Will See

Here are some of the most frequently encountered fault codes in modern diesel trucks:

SPN	FMI	Description	Common Cause	Severity
3251	0	DPF Soot Loading Excessively High	Missed regenerations, excessive idle time, failed DOC	High — can cause derate
3936	16	SCR Catalyst Conversion Efficiency Low	Bad DEF fluid, failed DEF injector, contaminated SCR catalyst	High — EPA derate
102	3	Intake Manifold Pressure Voltage High	Boost pressure sensor failure, wiring short to +V	Medium
520322	5	Aftertreatment Fuel Pressure Low (Open Circuit)	Broken wire to AT fuel pressure sensor, failed sensor	High — blocks regen
3226	0	AT Exhaust Temp Too Low for DPF Regen	Failed DOC, exhaust leak before DOC, low fuel pressure to 7th injector	High — prevents regen
1761	1	DEF Quality Poor (Below Normal)	Wrong DEF concentration, contaminated DEF, failed quality sensor	High — 5 mph derate
100	1	Engine Oil Pressure Below Normal	Low oil level, worn oil pump, oil filter restriction, bearing wear	Critical — engine protection
110	0	Engine Coolant Temperature High	Thermostat, water pump, radiator blockage, fan clutch failure	Critical — engine protection
157	3	Fuel Rail Pressure Voltage High	Sensor failure, wiring issue, fuel system mechanical fault	High
627	2	Battery Voltage Erratic	Alternator failing, loose battery connections, voltage regulator	Medium

SPN Ranges: Quick Reference

SPNs are organized into ranges by system. Knowing the range helps you quickly identify which system is affected:

SPN Range	System
0–999	Engine, drivetrain, vehicle systems
1000–1999	Transmission, brakes, electrical
2000–2999	Marine, ag, off-highway specific
3000–3999	Aftertreatment (DPF, SCR, DOC, DEF)
4000–4999	Aftertreatment extended, inducement
5000–5999	Body, lighting, HVAC
520000+	Manufacturer-specific (proprietary)

Pro Tip: If you see an SPN in the 520000+ range, it is a manufacturer-proprietary code. You will need the specific engine or truck manufacturer's service manual to decode it — these are not in the public SAE J1939 standard. A good heavy-duty scanner will still decode these if it has the manufacturer's enhanced database.

DM Messages: How Codes Travel on the Bus

SPN/FMI codes are carried inside DM (Diagnostic Message) types on the J1939 bus. The most important ones for everyday diagnostics:

DM1 (PGN 65226): Active fault codes — these are currently triggered. Broadcast every second by each ECU.
DM2 (PGN 65227): Previously active (stored) codes — the fault occurred but is no longer active. Sent on request.
DM3 (PGN 65228): Clear/reset command — sent by the scanner to erase codes.
DM5 (PGN 65230): Readiness status — similar to OBD-II readiness monitors.
DM11: Clear all active and previously active codes (used during service resets).
DM12 (PGN 65236): Emissions-related active codes (used for HD-OBD compliance).