How to Clear a Fanuc Soft Overtravel Alarm with P + CAN

A soft overtravel alarm means an axis has reached — or the control believes it is beyond — its stored stroke limit, the software travel boundary set in parameters. Most of the time you clear it by jogging the axis back the other way and pressing RESET. But if the axis is already past the limit when the control boots, the control can refuse to move it in either direction. The fix is to restart the control while holding the P and CAN keys, which skips the stored stroke check for that power-up so you can jog the axis back inside its working envelope.

Recognise the alarm

The number depends on the control generation:

The alarm screen also names the axis — for example OT0500 +OVERTRAVEL (SOFT 1) Z means the Z axis tried to go past its positive soft limit.

Hard overtravel is a different alarm (OT0506 / OT0507, or 506 / 507 on older controls). That one is triggered by a physical limit switch on the axis, and P + CAN does not touch it — see the section on hard overtravel below.

First, try the normal way

Before restarting anything, try this — it clears the large majority of soft overtravel alarms:

1. Switch to JOG or HANDLE (MPG) mode.
2. Jog the alarmed axis in the opposite direction to the alarm (a + alarm needs a − jog).
3. Press RESET.

If the axis moves and the alarm clears, you're done — no restart needed. Use P + CAN only when the alarm comes up immediately at power-on and the axis won't jog in either direction.

Why it happens at power-on

On machines with absolute pulse coders the control knows each axis's position the moment it powers up, and it checks that position against the stroke limits straight away. If an axis is sitting outside the limit — because it coasted over during an emergency stop, was moved by hand or with the brake released, or the soft limit parameters were changed — the control raises the alarm before you can do anything. The stroke check is now blocking the very move that would fix it.

Clearing it with P + CAN — step by step

1. Make the machine safe. Close the guards, clear the work area, and make sure nothing is clamped in a position that a slow axis move could hit. Note which axis and which direction the alarm names.
2. Switch the control OFF at the CNC power-off button (and the main isolator if your machine's procedure requires it). Wait a few seconds.
3. Press and hold the P and CAN keys on the MDI keypad together.
4. Switch the control ON while still holding both keys. Keep holding them until the position screen appears — typically a few seconds — then release.
5. The control boots without the stored stroke check. Release E-stop and reset any other alarms (such as an E-stop or hydraulic alarm) as usual.
6. Select HANDLE (MPG) mode at the ×1 or ×10 increment — or JOG at a very low feed override — and move the alarmed axis back toward the centre of travel, away from the direction in the alarm. Move it well clear, not just across the line.
7. Return the axis to its reference position (ZRN / home) if your machine uses incremental homing, or confirm the position readout is correct on an absolute-encoder machine.
8. Power-cycle the control normally (no keys held). The stored stroke limits are active again on this boot. Jog the axis gently toward the limit and confirm the alarm triggers where it should.

Safety: what you've switched off

While the control is running after a P + CAN start, the software limits are not protecting the axis. Only the hard limit switches and the mechanical end stops remain. So:

• Never run a program in this state — jog only.
• Use the handwheel at the smallest increment you can live with; a fast jog toward the wrong end can drive the axis into a hard stop, the tool into a fixture, or the turret into the chuck.
• Watch the axis, not the screen, while it moves.
• Don't leave the machine in this mode for the next operator — finish with a normal power-cycle.

If the alarm comes straight back

A soft overtravel that recurs after a clean restart is a symptom, not the fault itself. Check these before you reach for P + CAN again:

• Absolute encoder battery. A low or dead battery (alarm DS0306/306 battery low, or DS0300/300 APC alarm: need ZRN) loses the stored position, so the control's idea of where the axis is no longer matches reality. Change the battery with the control powered on so the position isn't lost, then re-establish the reference position if needed.
• Reference position lost or shifted. A crash, a coupling slip between motor and ballscrew, or a replaced motor/encoder moves the machine's zero — every soft limit is then offset by the same amount. Re-establish the reference position per the machine builder's procedure.
• Stroke limit parameters changed. On most Fanuc controls stored stroke limit 1 lives in parameters 1320 (+) and 1321 (−), in machine units, per axis. If someone edited them, the envelope has moved. Compare against the machine's parameter backup — don't guess new values.
• A program or offset driving past the limit. If the alarm happens mid-cycle, check the work offset, tool length/geometry offset and the program's coordinates — the limit did its job and stopped a move that was going to crash.

Hard overtravel is different

If the alarm is OT0506 / OT0507 (or 506 / 507) — or the machine shows an overtravel message from the PLC — the axis has hit a physical limit switch. P + CAN doesn't bypass it. Most machines have an overtravel release button (or a key switch) on the operator panel: hold it, jog the axis off the switch in the opposite direction, then press RESET. If there is no release button, the machine builder's manual covers the procedure. A hard overtravel always means the soft limits failed to stop the axis first, so find out why before carrying on.

Quick reference

• Soft OT while jogging or running → jog the opposite way, RESET.
• Soft OT at power-on, axis won't move → power on holding P + CAN, handwheel the axis back, home, power-cycle normally.
• Keeps coming back → check encoder battery, reference position and parameters 1320/1321.
• Hard OT (506/507) → overtravel release button + jog off the switch.

Key names and boot behaviour can vary slightly between Fanuc generations and how each machine builder configures the control, so if P + CAN doesn't take effect on your machine, check the builder's maintenance manual before changing parameters.

Looking at a used machine with a Fanuc control? Ask us about the control generation, alarm history and condition of any machine in our stock before you buy.