If you’re new to SAE J1939, one of the first things you’ll encounter is how heavy-duty vehicles report problems. They do this using standardized diagnostic messages, and the two most important are:

DM1 — Active Diagnostic Trouble Codes
DM2 — Previously Active Diagnostic Trouble Codes

Understanding these two messages will give you a solid footing in J1939 diagnostics and help you interpret what a vehicle is trying to tell you—both right now and historically.

What Are DM1 and DM2 Messages?

DM1 — Active Faults

DM1 is sent automatically when something is wrong right now.

“There is a problem that requires attention.”

As long as the fault remains active, the engine or ECU repeats the DM1 message roughly once per second.

Typical examples:

High coolant temperature
Low oil pressure
Sensor voltage out of range
Communication loss between ECUs

DM2 — Previously Active Faults

DM2 contains faults that happened earlier but are not active anymore.

“A problem occurred in the past, but it has cleared.”

DM2 is helpful for:

Diagnosing intermittent issues
Maintenance planning
Trend analysis
Warranty documentation

Unlike DM1, DM2 is transmitted only when specifically requested by a diagnostic tool.

What’s Inside a DM1/DM2 Message?

Both DM1 and DM2 contain one or more Diagnostic Trouble Codes (DTCs).
Each DTC includes:

SPN (Suspect Parameter Number): The item or value that failed
FMI (Failure Mode Identifier): How it failed
OC (Occurrence Count): How many times it has happened
CM (Conversion Method): How the SPN is encoded

DM1 additionally includes lamp status, which we explain next.

Understanding Lamp Status in DM1 Messages

One of the most useful parts of a DM1 message is its lamp status.
These lamps communicate the severity of a fault and how urgently it needs attention.

J1939 defines four primary diagnostic lamps:

1. MIL — Malfunction Indicator Lamp

Color: Amber
Meaning: A failure that may affect emissions or long-term engine performance.

Simplified:

“A performance or emissions-related issue exists. Pay attention soon.”

Common reasons include EGR faults, temperature/pressure sensor problems, or fuel control issues.

2. Amber Warning Lamp

Color: Amber
Meaning: A moderate problem that is not yet critical.

Simplified:

“There’s a problem, but the vehicle can continue operating for now.”

Examples include borderline sensor readings or early-stage component degradation.

3. Red Stop Lamp

Color: Red
Meaning: A severe, immediate issue that could cause damage or unsafe operation.

Simplified:

“Stop the vehicle as soon as safely possible.”

Common triggers include critical overheating, extremely low oil pressure, or major system failures.

4. Protect Lamp

Color: Amber or mixed (varies by manufacturer)
Meaning: The ECU is limiting power or entering a protective mode.

Simplified:

“The engine is reducing power to protect itself.”

This often accompanies derate conditions or limp-home operation.

Lamp States

Each lamp can be:

OFF
ON
Flashing

Diagnostic tools usually decode these states into readable text.

Lamp behavior helps technicians quickly determine:

Severity
Repair urgency
Whether the vehicle can continue operating

Why DM1 and DM2 Messages Matter

1. They guide troubleshooting

DM1 tells you which faults are happening now.
DM2 shows a history of past issues.

2. They help distinguish real-time vs intermittent problems

A DM2 entry may reveal:

Voltage dips
Loose wiring
Cold-weather sensor failures
Conditions that resolved before inspection

3. They are essential for fleet maintenance

DM1 prevents immediate failures; DM2 provides long-term patterns.

Together, they form a complete diagnostic picture.

How Diagnostic Tools Display DM1 & DM2 (General Explanation)

Most J1939 diagnostic or monitoring tools display DM1 and DM2 in a format similar to:

A list of SPNs with readable descriptions
FMI explanations
Occurrence counts
Lamp status (for DM1)
Source address of the reporting ECU
Timestamps

Different tools vary in layout, but the underlying information follows the J1939 standard.

This explanation avoids assuming features of any specific software.

Simple Real-World Examples

Example 1: DM1—High Coolant Temperature

SPN 110
FMI 15 — High temperature
Lamp: Amber Warning ON
Repeats every ~1 second until temperature returns to normal.

Example 2: DM2—Low Oil Pressure (Historical)

This could occur if:

Oil pressure dropped earlier
Engine was shut down
Oil was added
The active fault cleared

DM2 confirms the event occurred even if the vehicle now runs normally.

Key Takeaways

DM1 = Active faults occurring now
DM2 = Historical faults that cleared
Both contain DTCs describing what failed and how
DM1 includes lamp status indicating severity
DM1 is broadcast automatically
DM2 requires a request to retrieve

Understanding DM1 and DM2 is one of the most important early steps in learning J1939 diagnostics.

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