Boom Length Sensor Fault: What to Check

What the boom length sensor does

The boom length sensor tells the controller how far the telescoping boom is extended. Together with the boom angle it sets how much reach the machine is using, which is what the controller needs to keep the platform or load inside the machine's stability envelope. Many machines also have a retracted limit switch as a second, independent check that the boom is fully in.

What the machine does when it faults

On the Genie S-40/45 XC, the fault chart for the primary boom length sensor shows how seriously the controller takes it. If the controller loses the sensor entirely ("no CAN messages have been received"), boom up, extend and turntable rotate stop working. If the sensor's two internal channels disagree, the machine is "limited to recovery functions": you can bring the boom home, not keep working.

Fault type (Genie S-40/45 XC, primary boom length)	What it means	What the manual has you check
Not detected	The controller is getting no messages from the length sensor	Sensor power, then the CAN wiring: with the sensor unplugged, resistance across the two CAN pins should be 55 to 65 ohms
Cross check failed	The sensor's two internal channels report significantly different lengths	That the sensor's cord is hooked to its bracket on the boom and extends and retracts smoothly; then calibrate, then replace
Not calibrated	Calibration was never completed	Calibrate the boom length sensor
Value out of expected range	Length out of range, or the retracted limit switch disagrees with the length sensor	That the retracted limit switch changes state as the boom retracts and extends; the switch for physical damage; then calibrate

What to check first

1. Walk the boom. A length sensor that measures with a cord or cable reel only works if the cord is attached and runs freely; Genie's first check for a cross-check fault is exactly that. 2. Check the retracted limit switch and its arm for damage. When the switch and the sensor disagree, the controller cannot tell which one is lying. 3. Check connectors and the wiring along the boom, where cables flex every time the boom moves. 4. Recalibrate after any repair.

Calibration

On MEC's 66-RJ, boom length calibration is done from the ground-control menu with the machine on a flat, level surface, the main boom fully retracted and stowed, and the sensor reading 0 inches, then holding Set. Other machines use other procedures; use your machine's manual.

A note on codes

Fault codes are not shared between makers. Genie, JLG, Skyjack, MEC, Manitou and CAT each number their faults differently, and the same maker numbers them differently across control generations. The descriptions here explain the component and how it fails; the code your machine shows has to be looked up in that machine's own service manual, by model and serial number.

Frequently Asked Questions

Why will my boom lower and retract but not extend?

That pattern matches a controller that has limited the machine to recovery functions because it no longer trusts the boom length reading. Bringing the boom in is allowed; extending further is not, until the sensor is fixed or recalibrated.

What is a cross-check fault on a boom length sensor?

Some length sensors measure the same thing on two internal channels. If the two readings disagree by too much, the controller cannot know which is right and treats the reading as unsafe. Genie's chart lists checking the sensor cord and recalibrating before replacing the sensor.

Do I need to recalibrate after replacing the sensor?

Yes. Every maker's procedure ends with a calibration, normally with the boom fully retracted and stowed on level ground.

Sources

Documents this guide was checked against. How we verify.

Boom Length or Extension Sensor Fault: Why the Boom Won't Extend and What to Check