JLG Telescopic Boom Lift 86 Fault Code: Internal Temperature

What does JLG Telescopic Boom Lift error code 86 mean?

Fault code 86 is a shared code number that JLG uses for two very different conditions. One version relates to the control unit's own internal temperature running outside its permissible range. This is a control unit hardware fault, not a machine hydraulic or steering fault, and it clears on its own once the control unit's internal temperature returns to a normal range.

The other version, reported as SPN 171 / FMI 12, covers a long list of steering system and oscillating axle problems. This includes pressure switch disagreements on electrically released oscillating axles, short-to-battery or short-to-ground faults on axle extension and retraction valves, similar shorts on the front and rear left and right steer valves, decoupled steering angle sensors, wheels that are blocked and not responding to steer commands, and steering angle sensors reading out of range low or high.

Because the analyzer display shows the specific text (for example 'Axle EXT STB or OC' or 'Front left wheel blocked'), always read that exact message. It tells you which valve, sensor, or wheel position is involved, which narrows the diagnosis considerably.

Common causes of 86

• Control unit internal temperature outside its permissible operating range, which is a control unit hardware condition rather than a wiring fault.
• Pressure switch on an electrically released oscillating axle disagreeing with the machine's drive state (indicating pressure while not driving, or not indicating pressure while driving and restricted to transport).
• Short to battery or open circuit on the axle extension valve, axle retraction valve, or any of the front/rear left/right steer valves (right or left coil).
• Short to ground on the axle extension valve, axle retraction valve, or any of the front/rear left/right steer valves (right or left coil).
• Steering angle sensor decoupled from its mounting on the front left, front right, rear left, or rear right wheel.
• A wheel not responding to steer commands (blocked) at the front left, front right, rear left, or rear right position.
• Steering angle sensor signal out of range low (short to ground or open circuit) or out of range high (short to battery) at any of the four wheel positions.

How to troubleshoot JLG Telescopic Boom Lift 86: first checks

1. 1Note the exact analyzer text for the fault. There are over 30 distinct sub-messages under code 86, and the wording tells you whether the issue is temperature-related or tied to a specific valve, sensor, or wheel.
2. 2For the control unit temperature version, cycle the ignition off and back on, then check whether the fault returns. If it does, this points to a control unit hardware problem that should be referred to OEM service.
3. 3For steering or axle valve faults, inspect the wiring and connectors at the named valve (axle extension, axle retraction, or the specific steer valve coil) for corrosion, chafing, pinched wires, or loose pins before replacing any component.
4. 4For 'fault-check mounting' messages, physically inspect the steering angle sensor at the named wheel position to confirm it has not come loose or decoupled from its mount.
5. 5For 'wheel blocked' messages, check for mechanical obstructions or binding at the named wheel that would prevent it from responding to steer commands. These faults are noted to clear on their own once the blocking condition is resolved.
6. 6For sensor out-of-range messages, test the steering angle sensor circuit at the named wheel for a short to ground, open circuit, or short to battery depending on whether the message says low or high.

How the code clears

The control unit internal temperature version of fault 86 is described as a message that disappears once the control unit's internal temperature returns to the normal range. Cycling the ignition off and on again is the listed step; if the fault keeps returning, it should be referred to OEM service. The 'wheel blocked' steering messages are noted to clear automatically once the blocking condition clears. No separate clearing step is listed for the individual valve short or sensor range faults beyond correcting the underlying wiring or sensor issue.