JLG Boom Lift 21 Fault Code: Problems with Foot Switch
Also called Over Voltage, Over Voltage - Over 63VDC, Overvoltage, Overvoltage - Over 63VDC
Problems with Foot Switch · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 21 on JLG boom lifts (M600) covers two distinct issues sharing the same code: an over voltage shutdown (alternator output over 63VDC) and a foot switch (FSW) disagreement fault. Both trigger a system shutdown and need to be diagnosed separately.
High severity. This code results in a shutdown (RBS condition: Shutdown). Whether it is an over voltage event or a foot switch disagreement, the machine will not operate normally until the fault is cleared, and continued operation with a real over voltage condition risks damage to electrical components.
What does JLG Boom Lift error code 21 mean?
Fault code 21 is used for two different conditions on the M600. The first is an over voltage shutdown, where the alternator output voltage climbs over 63VDC. The controller measures this at the alternator output and delays the shutdown by a factory set period specifically so that a brief voltage spike (a transient) does not trip the machine unnecessarily. Only a sustained over voltage condition results in the shutdown.
The second condition tied to code 21 is a foot switch (FSW) problem, where the two foot switch signals do not agree with each other. This is treated as a fault requiring an EMS (presumably the machine's control/enable system) recycle to clear.
A related start-up scenario is also listed under this code: when neither EMS input is active, this can simply mean the system is switching on or discharging its capacitor bank, which is normal. However, JLG also notes that a welded (stuck closed) line contactor could cause this same symptom, so it is worth ruling out if the condition persists beyond a normal start-up.
What triggers a JLG Boom Lift 21 code?
Over voltage: alternator output voltage exceeds 63VDC, measured at the alternator output, with shutdown delayed by a factory set period to filter out transient spikes. Foot switch fault: the two foot switch input signals disagree with each other. Start-up condition: neither EMS input is active, which can be normal during power-up or capacitor bank discharge, or can indicate a welded line contactor.
Common causes of 21
- Alternator producing output voltage over 63VDC, causing a genuine over voltage shutdown
- Foot switch (FSW) wiring, contacts, or switch mechanism causing the two foot switch signals to disagree
- Normal power-up sequence or capacitor bank discharge where neither EMS input is yet active
- A welded (stuck closed) line contactor mimicking the neither EMS input active condition
How to troubleshoot JLG Boom Lift 21: first checks
- Check the alternator output voltage directly, as instructed in the OEM solution, to confirm whether it is exceeding 63VDC
- Inspect the foot switch and its wiring and connectors for damage, corrosion, or misalignment that could cause the two signal inputs to disagree
- Cycle the machine through a normal start-up and watch whether the fault clears on its own, since a brief neither-input-active state during power-up or capacitor discharge is expected
- Inspect the line contactor for signs of welding (contacts stuck closed) if the neither-EMS-input condition persists past normal start-up
- Recycle the EMS as specified if the fault is identified as a foot switch disagreement
How the code clears
For a foot switch (FSW) fault, JLG specifies that an EMS recycle is required to clear it. For the over voltage condition, no separate clearing step is listed beyond correcting the alternator output voltage; the shutdown is delayed by a factory set period to avoid nuisance trips from transients, so the fault should not reappear once the alternator is repaired and voltage is back within range. No reset procedure is listed for the start-up (neither EMS input active) scenario, since it is expected to resolve on its own during normal power-up unless a welded contactor is present.
Affected models and serial ranges
21 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 21 mean?
It covers two separate issues, an over voltage shutdown where alternator output exceeds 63VDC, and a foot switch (FSW) fault where the two foot switch signals do not agree with each other.
Why does my boom lift shut down with code 21 only sometimes?
The controller measures alternator output voltage and delays the shutdown by a factory set period specifically to avoid tripping on brief voltage transients. If the over voltage condition is momentary, the machine may not shut down every time.
How do I clear a foot switch related code 21?
JLG specifies that an EMS recycle is required to clear a foot switch (FSW) disagreement fault.
Is a code 21 at start-up something to worry about?
Not necessarily. If neither EMS input is active, this can simply mean the system is switching on or discharging its capacitor bank, which is normal. However, if this condition doesn't resolve, check for a welded line contactor.
What should I check first if I get an over voltage code 21?
Check the alternator output voltage directly. JLG's listed solution for this condition is to check alternator output voltage to confirm it is exceeding 63VDC.
Can a bad foot switch cause a shutdown even if voltage is normal?
Yes. Code 21 is shared between the over voltage condition and the foot switch disagreement fault, so a foot switch signal mismatch alone can trigger this code and the resulting shutdown.