Open any truck diagnostic conversation and J1939 and J1708 come up within a minute. These are the languages the truck’s control modules use to talk to each other — and to your laptop. Knowing which one a chassis speaks, how your adapter reaches it, and how faults are structured inside it is the difference between live data and a blank screen.
What a protocol actually is
A protocol defines how modules on the vehicle network format and exchange messages: the wiring and signalling (physical layer) and the meaning of the bytes (application layer). Your diagnostic adapter and software have to speak the same protocol the truck is using. Select the wrong one and the truck simply stays silent — usually with no error message that explains why.
J1708 and J1587: the legacy pair
These two standards work together. J1708 is the physical layer — a twisted-pair serial link — and J1587 is the message layer that defines what the data means. Together they were the backbone of heavy-duty diagnostics for years and were still in widespread use on trucks built into the mid-2010s, spanning the 2007–2016 emissions transition. Plenty of working trucks in that age range are still earning money today, which is why J1708/J1587 support remains a real requirement and not a legacy checkbox.
J1939: the modern CAN standard
J1939 is the CAN-based standard used on current heavy-duty vehicles. It carries far more parameters, updates faster, and is the backbone of modern truck diagnostics. Two details matter in practice.
Two baud rates — a very common no-comms cause
J1939 runs at either 250 kbps or 500 kbps. The older 250 kbps networks use the 9-pin Type I connector (black); 500 kbps networks, common from 2016 onward, use the 9-pin Type II connector (green). If your software or adapter is set to the wrong speed, you get nothing. Before pulling connectors apart, confirm the baud rate selection matches the truck.
PGNs, SPNs and the 29-bit identifier
J1939 messages are organised by PGN (Parameter Group Number), carried inside a 29-bit extended CAN identifier. Each PGN is a container for related parameters, and the individual signals — the SPNs — live inside those PGNs. This is why J1939 data feels so much richer than the older standard: a single broadcast message can carry several related values at once, and the structure is defined rather than manufacturer-invented.
Reading faults: SPN, FMI, and the older MID/PID/SID
On J1939 a fault is reported as an SPN (Suspect Parameter Number — which component or parameter) plus an FMI (Failure Mode Identifier — how it failed). FMI is a standardised set numbered 0–31, so the same failure mode means the same thing across makes. A few you will see constantly:
- FMI 3 — voltage above normal or shorted high
- FMI 4 — voltage below normal or shorted low
- FMI 5 — current below normal or open circuit
An SPN identifying a sensor circuit with FMI 5 points you at an open circuit rather than a failed sensor — that alone saves a part swap. On the older standard the equivalent is MID (which module), PID or SID (which parameter or component), plus the same FMI concept. Same logic, different numbering.
Connectors: 6-pin, 9-pin and 16-pin
Older trucks commonly use the 6-pin Deutsch; newer ones the 9-pin Deutsch. On the 9-pin, J1939 CAN sits on pins C and D and J1708 on pins E and F. Important caveat: a 9-pin port does not guarantee J1939 is present — verify the pinout rather than assuming. Some medium-duty and newer vehicles use the 16-pin OBD-II connector instead.
RP1210: the compatibility check buyers skip
Both OEM and all-makes software reach the truck through a Vehicle Diagnostic Adapter that complies with RP1210, the TMC standard interface between PC software and the adapter (a NEXIQ USB-Link is the common example). J2534 covers the pass-thru side used for emissions-related reprogramming. RP1210 compliance is the single biggest compatibility factor to check before buying: if the software expects an RP1210 driver and your adapter does not provide one, nothing else about the package matters. All-makes platforms such as NEXIQ eTechnician are built to cover both J1708/J1587 and J1939 through this kind of adapter, so one setup handles old and new chassis.
Why both protocols still coexist
Trucks last a long time. A workshop can put an older or transition-era truck (through the mid-2010s) on J1708/J1587 on the lift in the morning and a current J1939 truck in the afternoon. A setup that only supports J1939 will fail to connect to the older equipment. Practical differences to keep in mind:
- Data volume: J1939 carries far more parameters, so live data is richer and refreshes faster.
- Fault format: SPN/FMI on J1939; MID with PID or SID plus FMI on J1587.
- Physical connection: 6-pin on older chassis, 9-pin Type I or Type II on newer ones, 16-pin on some vehicles.
Standard data versus OEM-specific data
Manufacturers layer their own messages on top of the standard. A good all-makes tool reads the standard J1939 data on every truck and, on supported systems, a substantial amount of OEM-specific data and bidirectional testing as well — eTechnician is marketed with OEM-level bidirectional controls. Single-brand OEM software generally goes deepest on its own marque. The realistic view is coverage breadth versus single-brand depth, not “generic versus everything.”
When the connection fails
Work through the cheap checks first. Confirm the key is on and the connector is clean and fully seated. Confirm the correct protocol and baud rate are selected. Then measure the bus: J1939 is terminated by two 120-ohm resistors, so a healthy network reads roughly 60 ohms across CAN-H and CAN-L (pins C and D on the 9-pin) with the key off. A reading near 120 ohms suggests a missing terminator or a break in the bus; a very low or open reading points to a short or a disconnected segment. That one measurement resolves a large share of no-comms calls before anyone blames the software.
At DiagTechPro we supply professional diesel diagnostic software with a remote install and setup service over TeamViewer, so your adapter, drivers and protocol settings are configured and tested before you take the laptop to the truck. Browse our full range of diagnostic tools to find the right fit for your shop.
