Diagnostics concepts

Vehicle diagnostics in J1939-heavy networks often use DM1 (active faults) and DM2 (previously active) messages. Embedded32 exposes a subset for classroom fault observation and injection in simulation - not a full service bay tool chain.

DM1 mental model

flowchart TB
  ECU[ECU detects fault]
  LAMP[Lamp status bits]
  DTC[DTC records in payload]
  DM1[DM1 broadcast PGN]
  TOOL[Diagnostic tool / monitor]
  ECU --> LAMP
  ECU --> DTC
  LAMP --> DM1
  DTC --> DM1
  DM1 --> TOOL

Students learn:

How faults become broadcast messages
How a scan tool listens without polling every ECU individually
How lamp status relates to operator warnings

DTC structure (simplified)

A Diagnostic Trouble Code in J1939 combines:

Field	Purpose
SPN	What failed
FMI	Failure mode
Occurrence count	How often
Conversion method	Encoding variant

Embedded32 decode output names fields when the PGN is in the catalog - use simulation labs to correlate hex payloads with text.

Fault injection in labs

sequenceDiagram
  participant Inst as Instructor script
  participant Sim as sim ECU
  participant Bus as virtual CAN
  participant Stu as Student monitor
  Inst->>Sim: inject fault condition
  Sim->>Bus: DM1 frame
  Bus->>Stu: embedded32-tools monitor / decode

Lab 4 (labs/lab-04-diagnostics-and-faults/) walks through inject → observe → clear scenarios.

DM2 and clearing

DM2 carries previously active faults. Clearing faults in real ECUs involves legislated workflows; in Embedded32 simulation, clearing is a state reset on the simulated ECU for teaching - not a claim of regulatory compliance.

Relationship to OBD-II

OBD-II on passenger cars uses different PIDs and legislated modes. Embedded32 does not position its J1939 diagnostics subset as OBD-II. Courses covering both should label standards explicitly.

Tools

# Observe decoded traffic including DM PGNs when present
npx embedded32-tools simulate vehicle/basic-truck

# On Linux with vcan (optional hardware path)
npx embedded32-tools monitor vcan0

Limitations

Incomplete DTC database
No UDS/KWP2000 session layer
No certified scan-tool replacement