Code Details
| Display code | SPN 731 FMI 2 |
|---|---|
| SPN | 731 |
| FMI | 2 |
| OEM code | None listed |
| Manufacturer | ZF |
| System | Transmission �?gear selection |
| Component | Synchronizer / shift actuator / gear set |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The ZF TCU continuously calculates the actual gear ratio from the input and output speed sensors. If the ratio does not match the commanded gear, the TCU detects that the transmission has not engaged the intended gear �?either due to a synchronizer fault, an actuator not completing the shift, or speed sensor inaccuracy. This can cause the transmission to retry the shift or fall back to a safe gear. On trucks with ZF Astronic or Traxon transmissions, the clutch engagement sensor monitors clutch disc position for AMT shift control. ZF ETAS can perform a clutch calibration procedure that resets the engagement and disengagement reference points to account for clutch disc wear. A clutch that has not been recalibrated after a clutch replacement or after significant mileage accumulation may produce erratic shift engagement quality that eventually triggers sensor range faults. ZF ETAS clutch calibration is a routine part of the clutch replacement procedure and should be performed any time new clutch components are installed.
SPN 731 FMI 2 indicates the gear ratio data is erratic or does not correspond to the commanded gear. On ZF AS-Tronic and TraXon transmissions, shift actuators are pneumatically or electro-hydraulically controlled. ZF Testman provides commanded vs. actual gear position, shift actuator stroke data, and synchronizer engagement force parameters to identify the fault location.
Common Symptoms
- Gear engagement failure or retry during shifts
- Transmission may reject a specific gear or gear range
- Harsh or incomplete shifts at certain speed/load conditions
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Worn synchronizer rings unable to match shaft speeds for engagement
- Shift actuator not completing the full stroke for gear engagement
- Low air supply pressure reducing actuator force
- Speed sensor providing inaccurate ratio data
First Checks
- Connect ZF Testman or WABCO Toolbox and identify which specific gear is affected
- Check air supply pressure �?ZF shift actuators require adequate air supply for full stroke
- Monitor shift actuator stroke data in Testman during a shift attempt
- Compare input and output speed data to confirm ratio calculation accuracy
- Connect ZF ETAS and run the clutch calibration procedure to reset engagement reference points
- Check ZF ETAS clutch wear data to confirm current clutch disc thickness is within serviceable range
Can I Keep Driving?
The affected gear may be unavailable or unreliably engaged. The TCU typically avoids the problem gear and works around it. Diagnose to restore full gear availability.
Related Codes
Related Lookup Pages
Sources
- TOOLBOX PLUS Diagnostic Software ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence medium
Source: ZF Commercial Vehicle Solutions, TOOLBOX PLUS Diagnostic Software. 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
Can worn synchronizers on the ZF AS-Tronic be replaced without a full transmission overhaul?
ZF AS-Tronic and TraXon transmissions can be repaired by qualified ZF-authorized service centers. Synchronizer ring replacement is possible as part of a targeted repair, though accessing the synchronizers requires significant disassembly. ZF Testman can identify the specific affected gear to guide the repair scope.
How does ZF Testman identify which gear is causing SPN 731 FMI 2 on the AS-Tronic?
ZF Testman logs the gear number and shift event associated with each ratio mismatch fault occurrence. The fault history shows which specific gear consistently fails to achieve the correct ratio. This narrows the mechanical investigation to the actuator, synchronizer, and gear set associated with that particular position.
Will the ZF AS-Tronic or TraXon continue to drive if one gear has a ratio mismatch fault?
Yes, in most cases. The ZF TCU is designed to recognize a failed gear and adjust its shift strategy to avoid that gear if possible. The transmission will continue to operate with a reduced gear selection until the fault is repaired. Extended operation avoiding a gear does add load to adjacent gears and may accelerate wear.