Code Details
| Display code | SPN 1231 FMI 9 |
|---|---|
| SPN | 1231 |
| FMI | 9 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| System | Transmission �?communication |
| Component | J1939 CAN network / transmission control module |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The DT12 transmission communicates with the engine control module and other vehicle systems over the J1939 CAN network to coordinate gear selection, engine torque management, and shift timing. When this communication is interrupted or a required message is not arriving on schedule, the transmission logs this fault. The driver may notice erratic shifting, transmission warning lamps, or a fault mode that limits gear selection.
SPN 1231 is assigned to the J1939 Network 1 data link in the standard. FMI 9 indicates the specific message is not being received at the expected update rate �?the data has been absent for too long. On the DT12, this typically flags when the transmission control module cannot receive a required message from the engine MCM or another key vehicle ECU. The fault can originate from a break in the CAN High or CAN Low wire, a missing or failed 120-ohm terminating resistor, a connector fault, or a source ECU that has shut down or has a power supply fault.
Common Symptoms
- Transmission warning lamp or check transmission message on the dash
- Transmission may default to a limp-home mode with limited gear selection
- Erratic or delayed shifting behavior
- Other J1939 communication faults visible in DiagnosticLink across multiple modules
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Damaged, corroded, or broken J1939 CAN bus wiring (CAN High or CAN Low)
- Missing, failed, or incorrectly wired 120-ohm CAN terminating resistor
- Faulty connector or poor pin contact at the transmission control module or on the backbone harness
- A different ECU on the J1939 network that has failed and is no longer transmitting its messages
- Power or ground supply fault at one of the network nodes
First Checks
- Connect DiagnosticLink and check all active codes across all available modules �?widespread communication faults across multiple ECUs suggest a backbone wiring or terminating resistor issue
- Measure J1939 bus resistance between CAN High and CAN Low with all power off �?should read approximately 60 ohms (two 120-ohm resistors in parallel); a reading well above or below 60 ohms indicates a resistor or wiring fault
- Inspect the DT12 TCM harness connectors for bent pins, corrosion, or unseated terminals
- If only the DT12 is faulting (not other modules), trace the branch harness from the backbone to the TCM connector
- Check the MCM and TCM power and ground circuits �?a module that has lost power will stop transmitting its J1939 messages
Can I Keep Driving?
With J1939 communication faulted, the DT12 may operate in a default or limp-home mode that restricts available gears. The vehicle may still be driveable but with degraded performance. Continued operation with a CAN fault can mask other developing faults that require network communication to report. Diagnose the network issue promptly.
Related Codes
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium
Source: Detroit Diesel Corporation, DiagnosticLink. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - 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
FAQ
How do I measure J1939 bus resistance on a Detroit DT12?
With the ignition off and all control modules unpowered, measure resistance between the CAN High and CAN Low wires at the diagnostic port or at the transmission harness connector. A healthy bus with two 120-ohm terminating resistors reads approximately 60 ohms. A reading near 120 ohms suggests one resistor is missing or failed; a reading near zero suggests a short between CAN H and CAN L.
Can a faulty engine MCM cause SPN 1231 FMI 9 in the DT12?
Yes. If the engine MCM is not transmitting its expected J1939 messages �?because the MCM has a fault, lost power, or has shut down �?the DT12 TCM will flag an absent message fault for that source address. Checking which module is failing to transmit helps trace the fault to the correct ECU.
Will SPN 1231 FMI 9 on the DT12 clear itself if the CAN connection is restored?
Once the communication issue is resolved and the required messages are received consistently, the active fault will typically clear on its own during the next drive cycle. Stored codes can be cleared using DiagnosticLink after confirming the network is operating correctly.