Diagnosing Flash Code 9 on a JLG T350 Towable Boom Lift

[MikePhua]

The JLG T350 and Its Role in Aerial Access Equipment
The JLG T350 is a battery-powered towable boom lift designed for light-duty aerial work in construction, maintenance, and landscaping. With a working height of 40 feet and a platform capacity of 500 lbs, it offers a compact footprint and easy transportability. JLG Industries, founded in 1969 and now a subsidiary of Oshkosh Corporation, has long been a leader in aerial work platforms. The T-series towables were introduced to serve contractors and rental fleets needing quick setup and reliable reach without the bulk of self-propelled units.
By the mid-2010s, the T350 had become a staple in North American rental yards, with thousands of units deployed across schools, churches, and commercial facilities. Its electric drive system and simplified control layout made it popular among operators with minimal training.
Understanding Flash Code 9 and Its Implications
When a JLG T350 flashes code 9 on its control panel, it indicates a generic “Vehicle Requires Service” fault. This is akin to a check engine light on a car—alerting the operator that something is wrong, but offering no specifics. The machine may still function intermittently, but the fault prevents full operation and can lead to shutdowns mid-cycle.
Common symptoms include:

• Lift functions working briefly, then stopping
  
• Smooth operation followed by sudden inactivity
  
• No visible strain or overload during use
  
• Code 9 flashing consistently after startup
  

This fault is not tied to a specific subsystem, making it difficult to diagnose without a service analyzer—a proprietary handheld device used to read deeper diagnostic codes from the machine’s controller.
Potential Causes and Diagnostic Pathways
Several issues can trigger flash code 9:

• Faulty basket control cable or loose connectors
  
• Corrupted data in the controller memory
  
• Voltage irregularities from aging batteries or poor grounding
  
• Intermittent faults in the joystick or limit switches
  

In one case, an operator had recently swiveled the basket before the fault appeared. While this may seem unrelated, movement of the basket can stress the control cable or expose weak connections. Re-seating the cable at both ends resolved the issue temporarily, suggesting a contact fault rather than a hardware failure.
Recommended diagnostic steps include:

• Inspecting the basket control cable for wear, corrosion, or loose pins
  
• Unplugging and replugging connectors to refresh contact
  
• Checking battery voltage under load (should remain above 11.5V)
  
• Verifying ground paths from chassis to controller
  
• Using a JLG analyzer to retrieve specific fault codes
  

If the analyzer is unavailable, some technicians use voltage tracing and continuity tests to isolate faults manually. However, this requires a wiring diagram and familiarity with JLG’s control logic.
The Role of the Analyzer and Service Software
JLG’s service analyzer is essential for decoding flash codes beyond the basic numeric display. It connects to the machine’s controller and reveals:

• Specific fault codes (e.g., joystick out of range, sensor failure)
  
• Historical error logs
  
• Real-time voltage and signal data
  
• Configuration settings for calibration
  

Without the analyzer, troubleshooting becomes guesswork. In one Florida-based rental fleet, technicians used the analyzer to identify a recurring joystick fault that only appeared under full extension. Replacing the joystick resolved the issue and eliminated flash code 9.
Preventive Measures and Long-Term Reliability
To avoid flash code 9 and similar faults:

• Inspect control cables monthly and replace if frayed
  
• Clean connectors with dielectric grease to prevent corrosion
  
• Charge batteries fully before use and test under load
  
• Avoid pressure washing near electrical components
  
• Keep a log of fault codes and service actions
  

Adding a battery voltage monitor and surge protector can help prevent controller damage from low voltage or spikes. Some operators install aftermarket diagnostic ports for easier access, though this may void warranty coverage.
Field Anecdote from a Church Maintenance Crew
A maintenance team in Ohio used a JLG T350 to clean gutters and replace floodlights. After sitting idle for several years, the lift began flashing code 9 during use. The team unplugged and replugged the basket control cable, and the fault disappeared. They continued using the lift for weeks without issue, suggesting the problem was contact-related. Eventually, they replaced the cable and added a strain relief bracket to prevent future stress.
Conclusion
Flash code 9 on a JLG T350 is a catch-all fault that signals the need for deeper diagnostics. While the lift may still operate, ignoring the code can lead to unpredictable shutdowns and safety risks. With proper tools, clean connections, and preventive care, operators can resolve the issue and restore full functionality. In the world of aerial lifts, a flashing light is more than a nuisance—it’s a call to investigate before the platform stops mid-air.