JLG Articulating Boom Lift 666 Fault Code: CAN Bus Failure-Engine Controller
CAN Bus Failure-Engine Controller · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 666 sets when a CAN-configured engine controller stops sending CAN messages for more than 250ms. The ground module decelerates all functions and forces the engine state to stopped or mid-engine, and the operator must cycle the footswitch to get functions back.
High severity. This fault causes an immediate loss or restriction of function movement and shuts off the generator relay if equipped, so it stops normal operation until cleared. It is not a slow-developing issue, once CAN communication drops for the threshold time the machine reacts right away.
What does JLG Articulating Boom Lift error code 666 mean?
Fault code 666 is a CAN Bus Failure related to the Engine Controller. It only applies to machines where an engine with a CAN-based engine controller is configured in machine setup. The CAN bus is the communication network that lets the ground module and the engine controller talk to each other, sharing things like engine speed and status.
When the ground module stops receiving CAN messages from the engine controller for more than 250ms, it assumes something has failed in that communication path. Since it can no longer trust engine speed and status data, it takes a conservative approach: it decelerates all functions and either forces the engine state to ENGINE STOPPED, or if the engine was already stopped, it sets a target RPM of mid-engine and assumes the controller is reporting mid-engine.
If the machine is set up with a generator, the generator relay output is also shut off, and it will stay off until the operator re-enables it. In every case, the operator has to reactivate the footswitch before any functions will move again. This fault is the ground module's way of protecting the machine when it loses visibility into what the engine is actually doing.
What triggers a JLG Articulating Boom Lift 666 code?
The code sets when a CAN engine controller is configured in machine setup and no CAN messages are received from that engine controller for more than 250ms.
Common causes of 666
- A CAN engine controller is configured in the machine setup but is not actually communicating on the network.
- A broken, damaged, or disconnected CAN2 cable between the ground module (UGM) and the engine controller.
- Poor or corroded connections at the CAN2 tee connector or along the cable path to the engine.
- Wire continuity failure on individual conductors within the CAN2H or CAN2L wiring.
- Incorrect termination resistance on the CAN2 bus, which shows up as a resistance reading that is not approximately 120ohms.
How to troubleshoot JLG Articulating Boom Lift 666: first checks
- With power off, disconnect the CAN2 cable at the tee connector near the UGM.
- Using a multimeter, check resistance between the CAN2H and CAN2L pins in the path toward the engine controller, it should read approximately 120ohms.
- Check resistance between CAN2H and CAN2L in the path toward the UGM as well, again looking for approximately 120ohms.
- If either reading is off, check wire continuity on the individual CAN2H and CAN2L wires to isolate a broken or damaged conductor.
- Inspect the CAN2 tee connector and related connectors for corrosion, loose pins, or damage that could interrupt communication.
How the code clears
Once CAN messages are confirmed to be received from the engine controller again, normal operation is not automatically restored. The ground module requires the operator to reactivate the footswitch before functions will move again. No separate reset procedure beyond restoring CAN communication and cycling the footswitch is listed for this code.
Frequently asked questions
What does JLG fault code 666 mean?
It means the ground module has lost CAN communication with the engine controller for more than 250ms. This only applies to machines with a CAN-based engine controller configured in setup.
Why did my boom lift lose function movement after code 666 appeared?
The ground module deliberately decelerates all functions and forces a stopped or mid-engine state when it loses CAN communication with the engine controller, since it can no longer trust engine data.
Why is my generator relay off after this fault?
If the machine setup has GENERATOR=YES, the generator relay output is turned off automatically when this fault sets, and it will not turn back on until the operator re-enables it.
How do I clear code 666 and get functions back?
Restore CAN communication between the ground module and engine controller, then reactivate the footswitch. No functions will move until the footswitch is cycled, even after communication is restored.
What should I check first if I get this code?
With power off, disconnect the CAN2 cable at the tee near the UGM and check resistance between CAN2H and CAN2L pins on both the engine side and the UGM side. You are looking for approximately 120ohms in each path.
What if the resistance reading is not close to 120 ohms?
That points to a wiring problem. Check continuity on the individual CAN2H and CAN2L wires to find the break, corrosion, or damaged conductor causing the bad reading.