SICK C4000 Light Curtain — Red/Yellow LED & Weak-Signal Recovery

SICK · beginner · 20 min

Read the LEDs on a SICK C4000 safety light curtain to know what to do: red means the beam is broken or a fault is active, yellow means a reset is required, and weak signal points to alignment or contamination.

⚠️ A light curtain is a protective device. Never bypass, block, or defeat it. Clear the real cause of an interruption and only then reset — the field must be genuinely clear before the machine can restart.

Read the LEDs first

A SICK C4000 safety light curtain tells you its state through its indicator LEDs (and, on many variants, a 7-segment diagnostic display):

• Red — the protective field is interrupted or a fault is active. The OSSD safety outputs are OFF and the machine is signalled to stop. Red is asserted immediately when the beam is broken.
• Yellow — reset required. The interruption has been cleared but the system is waiting for a manual reset before it will allow the machine to restart (in configurations that use restart interlock).
• Green — the field is clear and the outputs are ON (in some configurations green appears only after an adjustable delay).

So before touching anything: red = something is in the field or faulted; yellow = it's clear, press reset; green = good.

Recovering from a red state

1. Find and remove the interruption. Something is breaking the beam between sender and receiver — a part, a pallet, an operator, or debris. Obstructions are not always clearly visible, so look carefully across the whole field, including low and high beams.
2. Check for a genuine fault vs. a blocked beam. If nothing is in the field but red persists, read the C4000's fault diagnosis / error display (the 7-segment code and the fault-diagnosis chapter of the operating instructions) to identify a device fault rather than an interruption.
3. Clear the yellow reset. Once the field is confirmed clear, the LED goes yellow (reset required). Press the machine's reset button for the safety function. The C4000 should switch to green and re-enable the OSSDs.

Weak signal / alignment problems

Intermittent red, a weak-signal warning, or a curtain that won't go green with nothing in the field usually means alignment or contamination, not a failed device:

1. Check alignment of sender and receiver — they must face each other squarely within tolerance. Vibration or an accidental knock can break alignment.
2. Clean the lenses of both sender and receiver, but do not disturb their alignment while cleaning.
3. Look for a partial obstruction or heavy contamination (dust, coolant mist, weld spatter) attenuating the beam.
4. Use the alignment aids — the C4000's alignment mode / signal-quality indication helps you re-aim the units until the received signal is strong across the field.

When to escalate

If a fault code persists after the field is clear, alignment is correct, and the lenses are clean, treat it as a device or wiring fault and consult the C4000 operating instructions or SICK support. Never wire out or block a light curtain to keep a machine running.