JLG Scissor Lift 661 Fault Code: CAN Bus Failure - Power Module
CAN Bus Failure - Power Module · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 661 on JLG M3369 and M4069 scissor lifts means the control system lost CAN bus communication with the power module. It points to a wiring, connector, or power supply problem between the ground control board and the power module, and it should be treated as a stop-and-diagnose issue before continuing to operate the lift.
High severity. This is a communication failure between the control system and the power module. The machine may not run correctly or at all until communication is restored, and a cycling line contactor can indicate a deeper wiring or power issue that should not be ignored.
What does JLG Scissor Lift error code 661 mean?
Code 661 fires when the machine's control system stops receiving messages over the CAN bus from the power module, which handles switching and power distribution for major functions on the lift. When this communication link drops, the control system loses visibility into what the power module is doing, and the machine can behave unpredictably or shut down functions to protect itself.
JLG's own help message summarizes it plainly: lost communication to the power module, with wiring at ground control and the power module identified as the likely fault area.
If the line contactor is cycling on and off, making an audible clicking noise when the machine is powered up, that is a specific symptom JLG ties directly to this fault, pointing at a power supply issue on the affected circuit.
Common causes of 661
- Loss of 24V supply between power module 12 position connector terminal 1 and the power module B- terminal, which can cause the line contactor to cycle on and off
- Broken or open continuity between ground board socket JI-31 and power module connector socket 10
- Broken or open continuity between ground board socket JI-32 and power module connector socket 11
- A related fault, DTC 662, present at the same time, which JLG says should be diagnosed first since it can be tied to the same failure
- General wiring or connector faults at the ground control board or power module, including corrosion, loose pins, or damaged harness
How to troubleshoot JLG Scissor Lift 661: first checks
- With the machine powered, check for 24V between power module 12 position connector terminal 1 and the power module B- terminal. Listen for the line contactor clicking on and off, which suggests this circuit is the problem
- Disconnect the ground board JI connector and the power module connector, then check continuity: ground board socket JI-31 to power module connector socket 10, and ground board socket JI-32 to power module connector socket 11
- Turn the machine on in platform mode and check whether DTC 662 is also present. If it is, troubleshoot 662 before continuing further on 661
- Inspect all connectors and wiring at the ground control board and power module for corrosion, loose pins, chafing, or moisture intrusion
- Confirm connectors are fully seated and locked, since a partially connected plug can look fine but still break CAN communication
How the code clears
JLG's help message states to recycle power to clear the fault after the wiring issue is corrected. No separate reset procedure is listed beyond cycling power to the machine.
Affected models and serial ranges
661 appears in our records across 2 JLG Scissor Lift models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| M3369 | Serial range not listed in source records |
| M4069 | Serial range not listed in source records |
Frequently asked questions
What does JLG fault code 661 mean?
It means the control system on the lift lost CAN bus communication with the power module. JLG describes it as a CAN bus failure tied to the power module and points technicians to check wiring at ground control and the power module.
Why is my JLG scissor lift's contactor clicking on and off?
A cycling line contactor making a clicking noise is a symptom JLG directly associates with code 661, typically pointing to a loss of the 24V supply between the power module's 12 position connector terminal 1 and the power module B- terminal.
Does fault code 661 clear itself?
No. JLG's help message says to recycle power to the machine to clear the fault once the underlying wiring or connection problem is fixed. There is no separate reset procedure listed beyond that.
Is DTC 662 related to fault code 661?
Yes. JLG specifically instructs checking for DTC 662 when the machine is turned on in platform mode, and if it is present, to troubleshoot that code before continuing with 661 diagnostics.
Which JLG models does fault code 661 apply to?
This fault code is documented for the M3369 and M4069 scissor lift models.
Can I keep operating the lift with a 661 fault active?
It is not recommended. A lost CAN link to the power module can affect how the machine controls its functions, and a cycling contactor suggests an electrical problem that could worsen. Diagnose and correct the wiring or connection issue before returning the lift to service.