JLG Telescopic Boom Lift 21 Fault Code: Problems with Foot Switch
Also called Help Comment - ADS 1213 Reinitialized, Over Voltage, Over Voltage - Over 63VDC, Overspeed, Problems with Footswitch
Problems with Foot Switch · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 21 on the JLG E600 telescopic boom lift flags a disagreement between the two foot switch signals, or a condition where both platform and ground mode are selected on the key switch at the same time. It also appears at startup when neither EMS input is active. This is a safety interlock fault, not a power or drivetrain failure.
Medium severity. The lift will not operate properly until the interlock disagreement is resolved, but this is a control/interlock fault rather than an immediate mechanical or electrical hazard. Treat it as a stop-and-diagnose issue before returning the machine to service.
What does JLG Telescopic Boom Lift error code 21 mean?
Fault code 21 is tied to the foot switch (FSW) and key switch interlocks on the JLG E600. The controller expects the two foot switch signals to agree with each other. If they read the same state for more than one second, or simply disagree, the controller flags this fault and requires an EMS (Emergency Stop / control system) recycle to clear.
A second trigger condition is the key switch: if both platform and ground modes are selected at the same time, the controller sees this as an invalid, contradictory input and sets the same fault code.
There is also a startup variant. If neither EMS input is active when the machine is powering on or the capacitor bank is discharging, the controller can log this fault. In this case a welded line contactor may also be the cause, since a stuck-closed contactor can mimic this condition.
What triggers a JLG Telescopic Boom Lift 21 code?
The controller sets this fault when the two foot switch inputs read the same state for more than 1 second, or when both platform and ground modes are selected simultaneously on the key switch. A startup variant is set when neither EMS input is active during power-on or capacitor bank discharge.
Common causes of 21
- Foot switch (FSW) signals do not agree with each other, or both read the same state for more than 1 second
- Key switch or keyswitch selects both platform and ground mode at the same time
- Neither EMS input is active during startup or while the capacitor bank is discharging
- A welded (stuck closed) line contactor, which can produce the same startup symptom as a missing EMS input
How to troubleshoot JLG Telescopic Boom Lift 21: first checks
- Inspect the foot switch and its wiring for damage, corrosion, or a stuck pedal that could cause both signals to read the same state
- Check the key switch for proper detent and confirm it cannot select platform and ground mode at the same time
- Cycle the EMS (recycle the control system) as specified for the FSW faulty condition, since this is explicitly called out as required to clear the fault
- With power off, inspect the line contactor for signs of welding or sticking, especially if the fault appears right at startup
- Confirm no other control inputs are conflicting or held active during power-up that could delay the capacitor bank discharge
How the code clears
For the foot switch disagreement fault, an EMS recycle is explicitly required to clear the code. No separate clearing step is listed for the key switch or startup variants beyond correcting the underlying condition (selecting only one mode, or allowing normal startup with a healthy EMS input and no welded contactor).
Affected models and serial ranges
21 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 21 mean on an E600 boom lift?
It means the controller detected a foot switch signal disagreement, both platform and ground mode selected at once on the key switch, or a startup condition where neither EMS input is active. All three are interlock-related faults, not drivetrain or engine faults.
Why do I need an EMS recycle to clear fault code 21?
When the fault is caused by the two foot switch signals disagreeing or reading the same state for more than 1 second, the system requires an EMS recycle before it will clear and allow normal operation again.
Can a bad key switch cause fault code 21?
Yes. If the key switch allows both platform and ground modes to be selected simultaneously, the controller treats this as an invalid state and sets fault code 21.
Does fault code 21 mean my foot switch is broken?
Not necessarily. It means the two foot switch signals do not agree, which can be due to a wiring fault, a stuck pedal, or an actual switch failure. Inspection of the wiring and switch mechanism is the first step.
Why does fault code 21 show up when I first turn on the machine?
At startup, if neither EMS input is active while the system is powering on or the capacitor bank is discharging, the controller can log this fault. A welded line contactor can also cause this same startup symptom.
Is it safe to keep operating the lift with fault code 21 active?
No. This fault involves the platform/ground mode and foot switch interlocks that are part of the machine's safety control logic. The lift should be diagnosed and the fault cleared before returning to normal operation.