JLG Boom Lift 31 Fault Code: Contactor Did Not Close When Energized
Contactor Did Not Close When Energized · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 31 on JLG M600 boom lifts means the traction drive controller energized a contactor (line, forward, or reverse) but did not see the expected electrical response. This points to a wiring error, a miswired contactor coil, or an open circuit in a contactor or the traction motor.
High severity. The machine's traction drive will not respond correctly or at all until this is fixed. Continuing to operate risks unexpected loss of drive control or unintended direction response, so treat this as a stop-and-diagnose issue before resuming travel.
What does JLG Boom Lift error code 31 mean?
Fault code 31 means the controller closed (energized) one of the traction contactors, either the line contactor, the forward direction contactor, or the reverse direction contactor, but did not see the electrical result it expected. On a normal system, closing the line contactor should raise the capacitor bank charge to match battery supply, and closing a direction contactor should make traction point A go high. When the controller doesn't see this happen, it raises code 31 to protect the traction system from operating with incorrect or missing power feedback.
There are four distinct conditions the controller checks for. If the line contactor closes and traction point A goes high instead of the capacitor bank charging up, the line and direction contactor coil wiring have likely been swapped. If either the forward or reverse contactor is energized and traction point A does not go high, that points to an open circuit in the traction motor, in that specific contactor, or a power wiring fault. If the line contactor is energized and the capacitor bank charge does not rise to battery supply, that indicates an open-circuit line contactor or a power wiring error.
Common causes of 31
- Line and direction contactor coil wiring swapped, so closing the line contactor produces the traction point A response instead of capacitor bank charging
- Open-circuit forward direction contactor
- Open-circuit traction motor affecting the forward direction circuit
- Open-circuit reverse direction contactor
- Open-circuit traction motor affecting the reverse direction circuit
- Open-circuit line contactor
- General power wiring error in the traction contactor circuits
How to troubleshoot JLG Boom Lift 31: first checks
- Visually inspect the line contactor and both direction contactor coils for correct wiring; look specifically for a swapped connection between the line contactor coil and a direction contactor coil
- With the machine safely isolated, check continuity through the traction motor circuit to rule out an open circuit
- Check continuity and physical condition of the line contactor, forward contactor, and reverse contactor, including their power wiring and connections
- Inspect all power wiring runs between the contactors, capacitor bank, and traction motor for loose, corroded, or broken connections
- Confirm capacitor bank charge behavior when the line contactor is energized, and confirm traction point A behavior when either direction contactor is energized, to match the fault pattern to one of the four listed conditions
How the code clears
No separate clearing step or reset procedure is listed for this code. Correcting the wiring error or repairing the open circuit that caused the fault is the documented fix; once the underlying contactor or wiring issue is resolved, normal operation should resume.
Affected models and serial ranges
31 appears in our records across 1 JLG Boom Lift models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| M600 | Serial range not listed in source records |
Frequently asked questions
What does JLG fault code 31 mean on an M600 boom lift?
It means the traction drive controller energized a contactor, either the line contactor or one of the direction contactors, but did not get the electrical feedback it expected. This usually points to a wiring problem or an open circuit somewhere in the contactor or traction motor circuit.
Is it safe to keep driving the lift with fault code 31 active?
No. This fault affects the traction drive's contactors, which control power to the traction motor and direction of travel. Continuing to operate risks unpredictable or absent drive response, so the machine should not be driven until the cause is found and fixed.
Can a wiring mistake during repair cause fault code 31?
Yes. One of the four listed conditions is specifically caused by the line contactor coil wiring being swapped with a direction contactor coil's wiring. This is worth checking first if the fault appeared after any electrical work on the contactors.
What is traction point A in relation to this fault?
Traction point A is a monitored electrical point in the traction contactor circuit that the controller expects to go high when a direction contactor closes correctly. If it doesn't go high when expected, the controller flags fault code 31.
Does fault code 31 always mean a bad contactor?
Not necessarily. It can also result from an open circuit in the traction motor itself or from a power wiring error, not just a failed contactor. All three possibilities need to be checked.
Is there a reset procedure for fault code 31?
No reset procedure is listed for this code. The fix is to find and correct the wiring or open-circuit fault; the code should clear once the underlying issue is resolved.