An electrical fault that is present all the time is often easier to diagnose than one that disappears as soon as the machine reaches the repair shop.
A scissor lift may work normally for several minutes, suddenly lose a function, then begin working again after the platform is lowered, a cable is moved, the machine cools down or the power is cycled.
These intermittent faults are frustrating, but the changing behavior is not meaningless.
In fact, when and how the fault appears can be one of the most useful diagnostic clues.
Common causes can include:
- Loose electrical connections
- Damaged connector terminals
- Broken conductors inside a harness
- Wiring affected by platform movement
- Heat-sensitive electrical components
- Moisture or corrosion
- Intermittent joystick or switch inputs
- Poor power or ground connections
This guide explains how to diagnose an intermittent scissor lift electrical problem systematically instead of replacing parts until the symptom disappears.
Important: Electrical systems vary by manufacturer, model and serial number. Diagnosis should follow the machine-specific wiring diagram, service manual and appropriate electrical safety procedures.
Intermittent Does Not Mean Random
When a fault comes and goes, it can appear random from the operator's perspective.
But there is often a condition that makes the problem appear.
That condition may be:
- Vibration
- Platform height
- Harness movement
- Temperature
- Moisture
- Electrical load
- Joystick position
- Machine movement
The first goal is therefore not simply to reproduce the fault.
The goal is to determine:
What changes immediately before the fault appears?
Record the Symptom Before Touching the Machine
Intermittent faults can disappear during inspection.
Before disconnecting connectors or cycling the machine, record as much information as possible.
Ask the operator:
- Which function stops working?
- Does the whole machine shut down or only one function?
- Does the fault happen when driving, lifting or both?
- Does it happen at a specific platform height?
- Does vibration make it appear?
- Does the machine recover immediately or only after waiting?
- Does cycling the emergency stop or power restore operation?
- Was the machine wet, recently washed or used outdoors?
- Were any fault codes displayed?
A vague report such as “sometimes it doesn't work” loses valuable diagnostic information.
Do Not Clear Fault Codes Too Early
If the machine displays a fault code while the problem is active, record it before cycling power.
Useful information may include:
- Exact fault code
- Function being commanded
- Platform position
- Whether the fault was active or stored
- What happened immediately before the code appeared
A diagnostic code may identify an affected circuit or signal even if the problem disappears later.
However, the code should be interpreted using the correct service documentation.
It is evidence—not automatic proof that one component must be replaced.
Vibration Is a Major Clue
If a fault appears while driving over rough ground or when the machine vibrates, inspect electrical connections carefully.
Vibration can expose problems such as:
- Loose connectors
- Weak terminal contact
- Broken connector locks
- Loose ground connections
- Internally damaged wires
- Poorly supported harnesses
The circuit may make contact when the machine is stationary, then open briefly as the machine moves.
A Connector Can Look Connected and Still Be Faulty
Do not judge an electrical connector only by whether the two halves are plugged together.
Inside the connector, individual terminals can have problems such as:
- Corrosion
- Reduced terminal tension
- Pushed-back pins
- Bent terminals
- Heat damage
- Moisture
The connector body may look normal while one circuit intermittently loses contact.
Inspect both the housing and the individual terminal condition where the service procedure permits.
For connector-specific identification, see our Aerial Lift Connector Identification Guide .
Broken Connector Locks Matter More Than They Look
A locking tab may seem like a minor plastic detail.
Its job, however, is to keep the connector fully seated while the machine vibrates and moves.
When the lock is broken:
- The connector can gradually separate
- Terminal contact can become inconsistent
- The fault can appear only during vibration
- Operation may return when the connector is pushed back together
A connector that cannot remain securely locked should not be treated as a reliable long-term connection.
Wiring Can Fail Inside Intact Insulation
One of the most difficult intermittent faults occurs when copper conductors begin breaking inside a wire while the outer insulation still looks normal.
Repeated flexing can gradually damage conductor strands.
The wire may conduct normally in one position and open when bent in another direction.
This is especially important in harness sections that move frequently.
Platform Height Can Reveal a Moving-Harness Fault
If the machine works correctly when lowered but loses a function as the platform rises, pay close attention to wiring that moves with the scissor structure.
Inspect for:
- Harness stretching
- Pinching
- Sharp bends
- Abrasion
- Broken cable supports
- Connector tension
- Contact with moving structure
A harness can be electrically connected at one platform height and lose continuity at another.
That makes platform position an important diagnostic variable.
Do Not Inspect a Moving Harness Only With the Lift Fully Lowered
A wiring harness can look perfectly routed when the machine is stored.
Its actual stress may only appear as the scissor mechanism extends.
Where the manufacturer's service procedure allows safe inspection, evaluate how the harness changes throughout the relevant movement range.
Look for sections that:
- Become tight
- Drag across structure
- Change bend radius sharply
- Pull against connectors
For complete harness identification and replacement considerations, see our Aerial Lift Wiring Harness Guide .
If Moving a Cable Restores Operation, Do Not Stop There
If the machine starts working when a cable or connector is moved, you have found an important clue—but not necessarily the exact failed point.
Movement may affect:
- The conductor inside the cable
- A nearby terminal
- The connector body
- A splice
- A ground connection
Do not assume the entire harness must be replaced until the affected area is identified.
Heat Can Make an Electrical Fault Appear
Some electrical problems become more noticeable after the machine has been operating for a period of time.
Electrical resistance and component behavior can change with temperature.
Useful questions include:
- Does the fault occur only after several minutes?
- Does operation return after the machine cools?
- Is a connector or terminal unusually warm?
- Does a motor controller fault appear under sustained use?
A machine that works perfectly cold but fails warm should be diagnosed differently from one that fails immediately after startup.
Warm Terminals Can Point Toward Resistance
Poor electrical connections can generate heat when current flows through excessive resistance.
Possible signs include:
- Terminal discoloration
- Melted connector material
- Darkened cable insulation
- Loose cable lugs
- Heat concentrated around one connection
A connection that becomes hotter under load may also create increasing voltage drop as the machine operates.
The underlying connection problem should be corrected—not only the heat-damaged component.
Moisture Can Create Intermittent Faults
An electrical problem that appears after rain, washing or outdoor storage may involve moisture.
Common areas include:
- Platform control box
- External connectors
- Switch assemblies
- Control boards
- Harness junctions
Moisture can cause temporary leakage paths, corrosion and unstable electrical contact.
A machine may begin working again after drying, but that does not mean the problem has repaired itself.
Corrosion or enclosure damage may remain.
Inspect the Platform Control Box for Water Entry
The platform control station is especially exposed to the operating environment.
Look for:
- Cracked enclosure
- Damaged cover
- Failed seal
- Condensation
- Water marks
- Corrosion around controls
- Moisture around internal connectors
If several internal components show corrosion, replacing one joystick or board without correcting the water-entry path may allow the failure to return.
An Intermittent Joystick Fault Can Be Position-Dependent
A joystick may produce a normal signal through part of its movement and lose that signal in another position.
Possible symptoms can include:
- Drive cuts out at one handle position
- Forward works more reliably than reverse
- The machine responds only after moving the joystick several times
- One function becomes intermittent while others remain normal
The joystick should be tested according to the machine's diagnostic procedure before replacement.
For joystick-specific failure symptoms, see: How to Identify a Faulty Aerial Lift Joystick .
Switch Contacts Can Also Fail Intermittently
A switch may move mechanically but make unreliable electrical contact internally.
This can affect components such as:
- Function selectors
- Emergency stops
- Key switches
- Limit switches
- Enable switches
If repeatedly cycling a switch temporarily restores operation, document the behavior and test the circuit rather than assuming the machine simply needed a reset.
Ground Connections Can Create Strange Symptoms
A poor ground can affect several circuits at once and create symptoms that appear unrelated.
Possible behavior can include:
- Random controller faults
- Intermittent control response
- Weak motor performance
- Different behavior under load
- Functions returning after vibration
Inspect major ground connections for:
- Loose hardware
- Corrosion
- Damaged cable
- Contaminated contact surface
A ground connection that looks attached may still have excessive resistance.
Battery Connections Can Create Intermittent Machine-Wide Faults
If multiple unrelated functions reset or disappear together, check the main power supply before focusing on one downstream component.
Inspect:
- Battery terminals
- Battery interconnect cables
- Main positive cables
- Main grounds
- High-current disconnect points
A brief interruption in main power can make several controllers or circuits appear to fail at once.
One Function Intermittent vs Whole Machine Intermittent
This distinction can dramatically narrow the fault.
If One Function Is Intermittent
Focus more closely on components unique to that function:
- Specific joystick signal
- Individual switch
- One wiring branch
- One connector terminal
- Function-specific output
If the Whole Machine Is Intermittent
Look more closely at shared systems:
- Main battery supply
- Ground
- Emergency stop circuit
- Control selector
- Main controller supply
The wider the symptom, the more likely the fault is located in a shared part of the electrical system.
Platform vs Ground Controls Can Narrow an Intermittent Fault
If the fault appears only when using the platform station, compare the same function from ground controls where applicable.
If ground operation remains reliable while platform operation is intermittent, focus more strongly on:
- Platform control box
- Platform joystick
- Platform emergency stop
- Platform connectors
- Moving platform harness
If both stations lose the function simultaneously, the fault may be farther downstream in a shared circuit.
See our Ground Controls Work but Platform Controls Don't Guide for a detailed comparison.
Load Can Expose a Connection That Looks Fine at Rest
A poor high-current connection may pass enough current for a meter reading or light-load condition but fail when the motor begins demanding more power.
Symptoms may include:
- Drive suddenly cutting out
- Pump motor slowing
- Controller resetting
- Voltage dropping significantly under load
Static voltage measurements alone may not reveal this type of fault.
Use the manufacturer's diagnostic procedure for loaded electrical testing.
Be Careful With the “Wiggle Test”
Gently moving wiring during diagnosis can sometimes help reveal an intermittent connection, but it should not become an uncontrolled test around moving or energized machinery.
Do not:
- Reach into the moving scissor structure
- Pull wiring while the machine is moving
- Bypass guards or safety devices
- Short terminals to create a response
Use safe access procedures and the machine's service instructions.
Previous Repairs Are Important Inspection Points
Intermittent faults often appear near areas that have already been repaired.
Inspect previous:
- Wire splices
- Crimp terminals
- Connector replacements
- Added jumper wires
- Harness repairs
A repair can look acceptable externally while having poor mechanical strength or electrical contact internally.
Do Not Replace a Control Board Just Because the Fault Is Electronic
Electronic symptoms can still originate outside the control board.
A board may appear to reset because of:
- Power interruption
- Poor ground
- Loose connector
- Communication wiring fault
- Moisture
Before replacing a PCB or controller, verify the power, ground and external connections that support it.
Reproduce the Conditions, Not Just the Fault
A powerful intermittent-fault strategy is to recreate the conditions under which the problem normally appears.
Examples include:
- Machine cold vs warmed up
- Platform lowered vs elevated
- Stationary vs driving
- Dry vs after moisture exposure
- Light load vs operating load
Do not create unsafe conditions simply to reproduce the fault.
Use only approved operating and service conditions.
Use a Simple Fault Pattern Log
For difficult intermittent problems, record each occurrence.
A useful log can include:
- Time
- Machine temperature
- Platform height
- Function commanded
- Fault code
- Weather or moisture condition
- Whether vibration was occurring
- What restored operation
After several events, a pattern may become obvious.
This is especially useful for rental fleets where the machine may be operated by different people.
Do Not Accept “Could Not Duplicate” as the End of the Diagnosis
If the machine works normally in the shop, use the operator report and fault history to identify likely trigger conditions.
Inspect the areas most consistent with those conditions.
For example:
- Fails elevated → inspect moving harness
- Fails over bumps → inspect connections
- Fails warm → investigate heat-sensitive circuits
- Fails after rain → inspect moisture exposure
- Fails under load → inspect high-current supply
The absence of the fault during one test does not prove the machine is repaired.
When Should a Harness Be Replaced?
Complete harness replacement may be appropriate when inspection confirms extensive or repeated damage such as:
- Multiple broken conductors
- Widespread corrosion
- Several previous repairs
- Burned wiring
- Extensive abrasion
- Damage across a moving harness section
A single damaged terminal or localized wire fault may have a different repair scope.
Follow approved repair procedures for the machine.
When Should a Connector Be Replaced?
Consider connector repair or replacement when there is confirmed damage to:
- Terminal retention
- Locking system
- Housing
- Pin alignment
- Wire entry
- Electrical contact surfaces
A connector that only works when held in a particular position is not a reliable repair.
A Practical Intermittent-Fault Diagnostic Sequence
1. Record the exact symptom.
Do this before resetting or disturbing the machine.
2. Identify the trigger condition.
Movement, vibration, height, heat, moisture or load?
3. Determine how much of the machine is affected.
One function or multiple systems?
4. Compare platform and ground controls.
Determine whether the fault is station-specific or shared.
5. Inspect connectors and moving harness sections.
These are common sources of position- and vibration-related faults.
6. Check main power and grounds.
Especially if multiple functions reset together.
7. Reproduce the normal trigger safely.
Confirm whether the symptom follows the condition.
8. Repair the confirmed fault and retest.
Do not stop because the symptom temporarily disappears.
Think in Conditions, Not Just Components
Intermittent troubleshooting becomes easier when the technician asks two questions together:
Which circuit is affected?
and:
Under what condition does it fail?
For example:
Platform control + elevated position may point toward a moving harness problem.
Whole machine + vibration may point toward main power or connection problems.
Drive + heavy load may point toward a high-current electrical problem.
Controls + moisture may point toward enclosure or connector contamination.
The combination of circuit and condition is often more useful than the symptom alone.
Frequently Asked Questions
Why does my scissor lift work sometimes and not others?
Possible causes include loose connectors, damaged wiring, intermittent switches, moisture, poor power or ground connections, heat-sensitive components or position-dependent harness damage.
Why does my scissor lift stop working when the platform is raised?
A platform-height-related fault can indicate wiring or connectors affected by harness movement, stretching, pinching or changing mechanical position.
Why does moving a cable make the lift work again?
Movement may temporarily restore contact in a damaged conductor, terminal, connector or splice. The exact failure point should still be identified and repaired.
Can moisture cause intermittent electrical problems?
Yes. Moisture can affect terminals, connectors, control boards, switches and wiring, and the symptom may change as the component dries.
Can a bad ground cause random fault codes?
A poor ground can create unstable supply conditions and unexpected electrical behavior. Ground integrity should be checked when several unrelated circuits are affected.
Should I replace the control box if platform controls fail intermittently?
Not automatically. The fault may originate in the joystick, connector, moving harness, power supply, emergency stop or other platform-side circuit.
Final Thoughts
An intermittent scissor lift electrical fault may seem unpredictable, but the conditions surrounding the failure usually provide valuable information.
Pay attention to:
Movement • Vibration • Height • Heat • Moisture • Load
Then identify which part of the machine loses operation under that condition.
The goal is not to wait until the fault becomes permanent.
It is to use the pattern of the intermittent failure to locate the circuit while the evidence is still available.
A problem that comes and goes should be treated as an active fault—not as a machine that has repaired itself.
Need Electrical Replacement Parts for a Scissor Lift?
SAIYIS supplies aftermarket AWP replacement parts including platform control boxes, joysticks, control boards, wiring components, switches, motor controllers and related electrical parts.
For compatibility assistance, provide the OEM part number, machine model, complete serial number and clear photographs of the original component, connectors and wiring.
Visit AWP-PARTS.COM to browse aerial lift replacement parts.
Manufacturer names and OEM part numbers are used for identification and compatibility reference purposes only. Electrical diagnosis and repair should follow the appropriate machine documentation.