JLG Telescopic Boom Lift 23 Fault Code: Problems with Boom Function Selection
Problems with Boom Function Selection · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 23 on JLG telescopic boom lifts (E600) covers a group of boom function selection faults, including joystick center tap issues, qprox sensor faults, and functions selected before the foot switch, key switch, start switch, or aux power. It is a priority (3) fault tied to how the controller validates joystick and pump switch inputs before allowing boom movement.
Medium severity. Priority (3) means the machine will typically lock out the affected boom function rather than cause immediate mechanical damage, but the platform or ground controls will not respond correctly until the fault clears. Treat it as a stop-and-diagnose issue before resuming lift operations.
What does JLG Telescopic Boom Lift error code 23 mean?
Fault code 23 is not one single failure. It is a family of related messages the JLG controller displays when it detects that a boom function (lift, swing, telescope, basket level, basket rotate, jib) was selected in the wrong sequence, selected in two directions at once, or reported an invalid voltage from the joystick circuit.
On machines with resistive joysticks, the controller watches the center tap voltage and the Vref reference voltage to confirm the joystick is wired and centered correctly. On machines with inductive joysticks, it instead watches qprox sensor voltage and centertap-based thresholds to confirm the joystick position is valid.
Several of the sub-faults are about sequencing: the operator or a stuck switch selected a boom function before the foot switch, key switch, start switch, or auxiliary power was engaged. Since JLG requires these functions to be enabled in the correct order as a safety interlock, any function detected out of sequence throws fault 23 and locks out that function.
What triggers a JLG Telescopic Boom Lift 23 code?
The exact set-conditions depend on which sub-fault is active. For inductive joysticks, the qprox sensor fault sets when qprox reads above 3.18 volts. Out-of-range-low on inductive joysticks sets below 1.05 volts when centertap is at the high end of its range, or below 0.79 volts when centertap is at the low end. Out-of-range-high on inductive joysticks sets above 4.35 volts at the high end of centertap range, or above 3.8 volts at the low end. Inductive center tap bad sets when centertap voltage is outside 2.18 volts to 2.70 volts. For resistive joysticks, out-of-range-high does not occur if Vref is below 8.1 volts; Vref above 7.7 volts is out of tolerance or indicates a short to battery. Resistive center tap bad sets when center tap voltage is outside 3.08 volts to 3.83 volts, with a tolerance band of about 0.1 volt around those limits. Pump switch and foot switch sequencing faults set when a function switch, foot switch, or aux power is detected closed before the required prior step (start switch, key switch, or foot switch) is closed.
Common causes of 23
- Joystick moved out of neutral (lift or swing) before or during foot switch closure
- A boom function switch has both directions (for example up and down, or extend and retract) selected at the same time, indicating a faulty pump switch
- A platform boom function switch was closed before the key switch or foot switch was closed
- A ground boom function switch was closed before auxiliary power was applied
- On inductive joysticks, the joystick moved out of neutral with no qprox sensors active, or a qprox sensor reading above the valid voltage threshold
- Joystick center tap voltage out of the valid range for the joystick type (resistive or inductive), often from wiring, connector corrosion, or a worn/failing joystick
- Vref reference voltage out of tolerance or shorted to battery on resistive joystick circuits
- A hydraulic function switch closed before the start switch was closed
- The foot switch was engaged when the operator attempted to start the machine
- Pump pot open-circuit, which drops most platform boom functions except upper lift and swing to creep speed
- Joystick wiper wire-off condition on lift, swing, or pump pot circuits
How to troubleshoot JLG Telescopic Boom Lift 23: first checks
- Confirm the operator is following correct start sequence: key switch, then foot switch, without touching any joystick or function switch beforehand
- Cycle the joystick to neutral and back, checking that it returns cleanly to center without binding
- Inspect joystick and pump switch wiring and connectors for corrosion, looseness, or chafing, especially at the center tap and qprox sensor wiring if equipped
- With a meter, check center tap voltage against the valid range for the joystick type installed (resistive vs inductive) and compare to Vref where applicable
- Check for a stuck or mechanically damaged pump switch that could be reporting both directions selected at once
- Verify aux power and key switch circuits are functioning and reaching the controller before any boom function switch is closed
- If equipped with inductive joysticks, check qprox sensor output and wiring for a short or bad ground that could push voltage above the fault threshold
How the code clears
No separate clearing step is listed. Correcting the underlying condition, such as returning the joystick to neutral before engaging the foot switch, releasing a stuck pump switch, or repairing an out-of-range joystick circuit, should allow the fault to clear on its own during normal operation. If it does not clear, treat it as a wiring or component fault and continue troubleshooting the specific sub-fault involved.
Affected models and serial ranges
23 appears in our records across 1 JLG Telescopic Boom Lift models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| E600 | Serial range not listed in source records |
Frequently asked questions
What does JLG fault code 23 mean?
It means the controller detected a problem with how a boom function was selected, either an out-of-sequence switch closure, a joystick center tap or qprox voltage out of valid range, or two directions of a function selected at once. It covers several related sub-faults under one code.
Why does my JLG boom lift say pump switches faulty or check diagnostics boom?
This sub-fault means a boom function switch, such as lower lift, telescope, basket level, basket rotate, or jib, has both directions selected at the same time. This usually points to a stuck or damaged switch.
Why does fault 23 show up when I try to start the machine?
If the foot switch is engaged when you try to start, or if a hydraulic function switch is closed before the start switch, the controller will flag this as an out-of-sequence condition and set fault 23.
Does fault code 23 mean my joystick is bad?
Not always. It can mean the joystick center tap voltage is out of range, which does often point to a joystick or wiring problem, but it can also be triggered by sequencing issues that have nothing wrong with the joystick itself, like moving it before the foot switch is closed.
What is a qprox sensor and why does it matter for this fault?
Qprox sensors are used on inductive joysticks to detect joystick position. If no qprox sensor is active when the joystick is moved out of neutral, or if a qprox sensor reads above its valid voltage, the controller sets fault 23 because it cannot confirm the joystick position is genuine.
Will fault code 23 stop my boom lift from working?
It typically locks out the specific boom function involved rather than shutting the whole machine down, though a pump pot open-circuit sub-fault will drop most platform boom functions to creep speed except upper lift and swing.
Is fault code 23 the same on resistive and inductive joystick machines?
No. Several sub-faults, like qprox sensor faults, only occur on inductive joysticks. Out-of-range-low and out-of-range-high faults behave differently or do not occur at all depending on whether the machine has resistive or inductive joysticks.