JLG Telehandler 6638 Fault Code: Communications - CAN Bus Failure - LSI
Also called CAN Bus Failure - LSI
Communications - CAN Bus Failure - LSI · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Code 6638 sets when the ground module (UGM) loses CAN bus communication with the LSI (Load Sensing Indicator/Load Moment Indicator) module for 1 second (1000 milliseconds). The machine defaults to LSI cutout behavior, treating the situation as if the load moment cutout percentage has been reached, and restricts operation to creep mode via the LSI cancel switch unless already in passive mode.
High severity. This code is treated as a machine system distress condition. The controller forces the machine into a load-moment-cutout style response, restricting normal lift functions to creep mode, so it directly limits safe operation until communication is restored.
What does JLG Telehandler error code 6638 mean?
Fault code 6638 covers CAN Bus Failure on the LSI system, described in two related ways: as a general CAN bus failure with LSI, and as a communications failure between the ground module and the LSI module. In both versions the core issue is the same: the ground module (referred to as the UGM in some documentation) stops receiving messages from the LSI module over the CAN bus network.
The LSI system is the load-sensing or load moment system that tells the machine how close it is to a tip or overload condition. When the ground module cannot hear from that module at all, it cannot trust the load data, so it defaults to a safe but restrictive state: it acts as though the load moment cutout percentage has already been reached, regardless of the actual input status, and it reacts according to how the machine's market configuration is set up.
Operation is not fully blocked. The machine allows continued movement only through creep mode, activated with the LSI cancel switch, as long as the machine is not already in passive mode. This gives the operator a controlled way to move the machine slowly while the underlying CAN communication problem is fixed.
What triggers a JLG Telehandler 6638 code?
The fault sets when the ground module (UGM) has received no CAN bus messages from the LSI module for 1 second (1000 milliseconds).
Common causes of 6638
- Loose, corroded, or damaged wiring or connectors in the CAN bus circuit between the ground module and the LSI module
- A failed or unpowered LSI module that is no longer transmitting on the CAN bus
- Damaged, pinched, or chafed CAN bus wiring causing intermittent or total loss of signal
- A blown fuse or interrupted power supply to the LSI module
- A software or internal fault within the ground module (UGM) preventing it from receiving or processing CAN messages
How to troubleshoot JLG Telehandler 6638: first checks
- Check that the LSI module has power and ground, and confirm its indicator or status light behaves normally if equipped
- Inspect all CAN bus wiring and connectors between the ground module and the LSI module for corrosion, looseness, or physical damage
- Verify connector pins are fully seated and not pushed back or spread, especially at the ground module and LSI module connectors
- Check fuses associated with the LSI module and ground module circuits
- Cycle the key and observe whether the fault clears once CAN communication is reestablished, which confirms an intermittent wiring or connection issue rather than a failed module
How the code clears
This fault is not latched in the sense of requiring a manual reset procedure in every version: one description says the machine reverts to normal operation automatically once CAN bus communication is reestablished. Other documentation states the fault is latched until a power cycle. In practice, restore CAN communication first, then cycle machine power to confirm the fault clears and does not immediately reset.
Frequently asked questions
What does JLG fault code 6638 mean?
It means the ground module lost CAN bus communication with the LSI (load sensing/load moment) module for 1 second (1000 milliseconds) or longer. The machine responds by defaulting to a load-moment-cutout style restriction.
Can I still operate the telehandler with code 6638 active?
Limited operation is possible. The machine allows creep mode movement using the LSI cancel switch, as long as it is not already in passive mode. Full normal operation is not available until CAN communication is restored.
Does code 6638 clear itself or does it need a manual reset?
It depends on the documentation version. One description says the machine automatically reverts to normal operation once CAN bus communication is reestablished. Another states the fault is latched until the machine's power is cycled. If the fault persists after a power cycle, the underlying wiring or module problem still needs to be fixed.
What usually causes CAN bus failure with the LSI module?
Common causes include damaged or corroded wiring and connectors in the CAN bus circuit, a failed or unpowered LSI module, chafed or pinched CAN wiring, a blown fuse, or an internal fault in the ground module itself.
Why does the machine act like the load moment cutout has been reached?
Because the ground module can no longer trust or receive load data from the LSI module, it defaults to the safest assumption: that the load moment limit has been hit. It reacts according to the machine's market configuration regardless of the actual state of the related input.
Is CAN bus failure with the LSI module dangerous to ignore?
It should not be ignored. The machine is flagging a loss of trusted load-sensing communication and indicating machine system distress. Continued operation is restricted to creep mode for a reason: the system cannot verify load conditions normally.