What J1939 Terminating Resistors Do
A J1939 CAN network requires exactly two 120-ohm terminating resistors placed at the physical ends of the backbone cable. These resistors prevent signal reflections — without them, electrical signals transmitted by one module travel to the end of the cable, reflect back, and collide with subsequent signals, causing bit errors that every module interprets as communication failures.
The two resistors in parallel give a target network impedance of approximately 60 ohms, measurable between the CAN Hi and CAN Lo pins at the 9-pin diagnostic connector with the ignition off. This 60-ohm measurement is a rapid network health check that takes less than one minute.
Terminator Fault Codes
There is no dedicated SPN for a missing or failed terminator. Instead, terminator problems appear as communication faults from multiple modules simultaneously — typically multiple FMI 9 (abnormal update rate) codes from different source addresses. Each module is reporting that the others have gone silent, when in reality the signal quality on the bus is too poor for reliable communication.
A resistance measurement well above 60 ohms (approaching 120 ohms) suggests one terminator is missing or failed; significantly below 60 ohms suggests an extra load or a short across the bus.
When Terminator Issues Typically Occur
Terminator faults most commonly occur after modifications to the J1939 backbone — adding or removing aftermarket devices (ELD units, telematics hardware, collision avoidance sensors), replacing a module whose connector contains a built-in terminator, or after wiring repairs that inadvertently affect the backbone ends.
Some aftermarket add-on devices include a built-in CAN terminator that, when connected to a properly terminated truck network, creates a third 120-ohm terminator — reducing the total impedance to 40 ohms and causing signal integrity problems.
Recording Guidance
Record whether the communication faults appeared after any recent addition, removal, or modification of J1939-connected devices. Perform the 60-ohm resistance check at the 9-pin connector and record the result before and after any suspected modification.
For aftermarket devices, check the device manufacturer's documentation for whether the device includes an internal CAN terminator.
Safety Context
A severely compromised J1939 network can cause modules to lose communication with each other, affecting ABS, stability control, and engine protection systems. Address multiple simultaneous FMI 9 codes promptly — they may indicate a network integrity problem rather than multiple independent module failures.
Related Pages
Sources
- 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 - Cleaner Trucks Initiative and Heavy-Duty Engine Emissions Context United States Environmental Protection Agency · government · accessed 2026-05-05 · confidence medium
Source: United States Environmental Protection Agency, Cleaner Trucks Initiative and Heavy-Duty Engine Emissions Context. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
What is the correct resistance measurement for a J1939 Terminating Resistor check?
With the ignition off and all modules connected, measure resistance between CAN Hi and CAN Lo at the 9-pin diagnostic connector. Two 120 Ω resistors in parallel give a target of approximately 60 Ω. Significantly above 120 Ω suggests a missing or failed terminator; significantly below 50 Ω suggests an extra bus load or a short circuit. Note: active modules affect the reading, so measure with ignition off for the most accurate baseline.
Can a missing termination resistor cause multiple modules to log communication faults?
Yes. Without correct termination, signal reflections on the CAN bus cause bit errors that every module interprets as communication failures from other modules. Multiple FMI 9 (abnormal update rate) codes from different source addresses appearing at the same time is a classic symptom of incorrect or missing bus termination. Check termination resistance early when multiple simultaneous communication codes are present.
Where are J1939 Terminating Resistors located on a commercial truck?
The two 120 Ω terminators are at the physical ends of the CAN backbone. On most commercial trucks, one is inside the ECM or ECM connector, and the other is at the far end of the backbone — often inside the instrument cluster, body controller, or a backbone junction block. Some aftermarket-installed modules have a built-in terminator that can add a third resistor if not accounted for in the network design.