JLG Articulating Boom Lift 21 Fault Code: Problems with Foot Switch
Also called Foot Switch Faulty, Key Switch Faulty, Overspeed, Overvoltage, Overvoltage - Over 63VDC, Problems with Footswitch
Problems with Foot Switch · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 21 on JLG articulating boom lifts covers several distinct conditions grouped under one code number: a foot switch signal disagreement, both platform and ground modes selected at once (key switch fault), and startup/overvoltage conditions. The controller flags this when the two foot switch inputs read the same state for more than 1 second, when both operating modes are active simultaneously, or during certain startup/discharge conditions. An EMS (Emergency Stop/reset) recycle is required after a foot switch fault.
Medium severity. This is priority (2) on JLG's fault scale. It typically prevents normal machine function (drive or lift operations tied to the foot switch or mode selection) until corrected, but it is not itself reported as an immediate catastrophic failure. Treat it as a stop-and-diagnose condition before returning the machine to service.
What does JLG Articulating Boom Lift error code 21 mean?
Fault code 21 is used by JLG for more than one underlying problem, so the first step is figuring out which body applies to your machine. The most common trigger is a foot switch disagreement: the two foot switch inputs, wired at connectors X169.8 and X007.15, are supposed to always read opposite states (one active, one not) whenever the platform footswitch is pressed or released. If both inputs read the same state for more than 1 second, the controller assumes the switch or its wiring is faulty and sets code 21.
A second version of this fault fires when both platform mode and ground mode are selected at the same time, through inputs at X007.2 and X007.3. Since a lift should only ever be in one control mode at a time, this dual-selection state points to a bad key switch (X030), damaged wiring, or failed diodes at the power down relay (X011).
A third scenario applies during startup: if neither EMS (foot switch enable) input is active while the system is powering on or discharging its capacitor bank, the controller can also log this fault. In this case a welded line contactor is called out as a possible cause. Regardless of which version applies, JLG requires an EMS recycle to clear the foot switch version of this fault.
What triggers a JLG Articulating Boom Lift 21 code?
The controller sets this code when the two foot switch inputs (X169.8 and X007.15) read the same logic state for longer than 1 second, when both platform (X007.2) and ground (X007.3) mode inputs are active at the same time, or when neither EMS input is active during machine startup or capacitor bank discharge.
Common causes of 21
- Foot switch itself is worn, sticking, or mechanically failing to fully release or engage
- Wiring faults (shorts or open circuits) in the foot switch circuit at X169.8 or X007.15
- Bad key switch (X030) allowing both platform and ground modes to be selected at once
- Damaged or worn wiring between the key switch and mode select inputs (X007.2, X007.3)
- Failed diodes at the power down relay (X011), allowing both mode signals through simultaneously
- Welded line contactor holding power in a way that prevents proper EMS input state at startup
- Startup or capacitor bank discharge sequence in progress when neither EMS input is active yet
How to troubleshoot JLG Articulating Boom Lift 21: first checks
- Confirm which fault body applies by checking the analyzer text: foot switch disagreement, key switch/mode conflict, or startup condition
- Inspect the foot switch mechanically for sticking, damage, or failure to release fully, and cycle it by hand while watching inputs on the analyzer if available
- Check foot switch wiring and connectors at X169.8 and X007.15 for corrosion, chafing, shorts, or open circuits
- Check the key switch (X030) for proper mechanical operation and confirm it cannot select platform and ground modes at the same time
- Inspect wiring at X007.2 and X007.3 for damage or crossed connections
- Inspect the power down relay (X011) and its diodes for failure
- If the fault appears only at startup, check the line contactor for signs of welding or sticking contacts
How the code clears
For the foot switch disagreement version, JLG specifically states an EMS (reset) recycle is required after the fault condition is corrected. No separate clearing step is listed for the key switch or startup/overvoltage versions beyond correcting the underlying wiring, switch, or contactor issue.
Frequently asked questions
What does JLG fault code 21 mean?
It means the controller detected one of a few possible problems grouped under this code: the two foot switch signals disagreeing for more than 1 second, both platform and ground modes being selected at the same time, or an abnormal EMS input state during startup or capacitor discharge.
Why does my JLG boom lift show a foot switch fault after I press the pedal normally?
The two foot switch inputs are wired to always be opposite each other. If they read the same state for more than 1 second, whether both on or both off, the controller assumes a wiring or switch fault rather than normal operation, and logs code 21.
Does fault code 21 need a reset after I fix it?
Yes, for the foot switch version JLG specifically calls for an EMS recycle after correcting the fault. The key switch and startup-related versions do not have a separate listed clearing step beyond fixing the cause.
Can a bad key switch really cause fault code 21?
Yes. If the key switch allows both platform and ground modes to be selected simultaneously, or if wiring or the power down relay diodes fail in a way that mimics this, the controller sets fault code 21.
Is fault code 21 safe to keep driving or lifting with?
No. This fault points to a problem in the foot switch or mode selection circuitry, both of which are core safety interlocks. Diagnose and correct it before returning the machine to normal operation.
What is a welded line contactor and why does it matter for this code?
A welded line contactor is one whose contacts have fused together, so it will not open and close normally. This can prevent the EMS inputs from reaching their expected state at startup, which the controller can misread as a fault condition and log as code 21.