P2643Torque Management Feedback Signal B Low
- Generic OBD-II code · Powertrain
- Can trigger limp mode
- How the car detects it
- With P2643 the transmission controller sees feedback signal B below its lower limit, so it cannot confirm that the engine is holding the torque level it asked for. No manufacturer describes this exact code in public documents. Where the feedback is a data-link message, as with Allison transmissions behind Cummins, Mack, Volvo or Navistar engines, a low value tends to come from a missing or defaulted message, an incompatible calibration or a bus fault; on a wired signal a short to ground is the classic cause. CARB requires comprehensive components to be tested against their range limits. Because Allison ties request B to Low Range Torque Protection, the TCM may lock out low ranges when it cannot verify the limit. P2644 is the high counterpart.
- What you may notice
- Transmission warning lamp on
- Low ranges or reverse low not selectable
- Harsh shifts because torque reduction is missing
- Likely causes
- LikelyTorque message missing or sent with a default value on the data link
- PossibleEngine and transmission calibrations incompatible
- RareBus or signal wiring damaged, short to ground
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €80 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.