# Litter-Robot 4 cat sensor fault: not detecting the cat, or detecting one that isn't there

A Litter-Robot 4 cat sensor fault usually comes from extra litter, bad placement or dusty curtain sensors, which make the scale and lasers misread. Whisker's fix is to remove litter above the MAX line, zero the scale, clean the bezel sensors and move the unit to a firm, level spot.

Updated Oct 27, 2026. HTML version: https://robotsforthat.com/guides/litter-robot-4-cat-sensor-fault/

## Short answer

A flashing red light bar means the Litter-Robot 4 has seen a cat or weight for over 30 minutes, usually from litter above the MAX fill line, a mat or cord under a foot, or the unit touching a wall. Dusty curtain sensors in the bezel cause both false detections and missed cats. The quickest fix is to remove extra litter, press Reset twice at Home to zero the scale, and wipe and vacuum the bezel. Cats under 3 lbs are not detected in Automatic Mode.

| What you see | Likely cause | Fix |
|---|---|---|
| Flashing red light bar | Cat or weight detected for over 30 minutes | Remove litter above the MAX fill line, then press Reset twice at Home. The light bar flashes white as the scale zeroes. |
| Blue with 2 flashing yellow | Scale triggered by extra weight or placement for over 30 minutes | Remove extra weight and move the unit off carpet, mats, cords and grout lines and away from walls, then press Reset. |
| Stuck mid cycle with a cat detected | Scale needs zeroing during the cycle | Long press Reset for 3 seconds. The unit zeroes and finishes the cycle. |
| Red and blue alternating | Dust or fur on the three curtain sensors | Wipe the bezel with a dry cloth and vacuum its inside edge with a brush attachment, passing over the holes at the top. |
| Green with 2 flashing red | Sensor auto calibration failed | Clean the curtain sensors, using a dry cotton swab and compressed air if needed, then press Reset and Cycle. |
| Yellow with 2 flashing red | Laser sensors need cleaning and firmware needs updating | Clean the sensors, then update the firmware. |
| Cat weight shows 0.0 | Dust or fur on the bezel blocks the laser sensors | Clean the bezel sensor area and press Reset. |
| Kitten visits not detected | Cat is under 3 lbs | Whisker says cats must weigh 3 lbs or more to use the unit safely in Automatic Mode. |
| Visits logged to the wrong cat or as Unknown | Cats within 1 lb of each other, old pet profiles, or the learning period | Update each cat's weight under Pets, reassign wrong visits, and allow up to 7 days for SmartScale to learn. |
| Blue and yellow button lights, 3 solid and 2 flashing | Robot or scale feet are unbalanced | Move the unit to level ground. |
| Red and green alternating light bar | Factory scale calibration is lost | Whisker says the base needs replacing. Submit a support request. |

## Step by step checks, in order

The Litter-Robot 4 uses a weight scale in the base and three laser curtain sensors in the bezel to decide if a cat is inside. When either reads wrong, the unit either refuses to cycle because it thinks a cat is there, or ignores a real visit. Whisker says a blue light bar after a step means it is fixed, and a flashing red one means move on to the next step.

- Confirm the unit is plugged in, online and on current firmware.

- If litter sits above the raised MAX fill line, scoop some out until it sits just below the line.

- From the Home position, press Reset twice to zero the scale. Do not lean on the unit while you press it, or your weight affects the reading.

- Wipe the bezel sensor area with a dry, clean cloth and vacuum the inside edge of the bezel with a brush attachment.

- Run a test: short press Reset, then short press Cycle.

- Check placement: a firm, level floor, all four feet moving freely, no cords or grout lines under the feet, the litter mat fully in front and every side clear of walls. On carpet, Whisker suggests the Litter-Robot 4 Carpet Tray.

- Press Reset again after you move the unit, add or remove litter, empty the drawer or clean it.

## Deep cleaning the curtain sensors

If the light bar is still not blue after a wipe, Whisker's laser sensor guide goes deeper. Turn the unit off at Home and unplug it. Press the latches on both sides of the bonnet, lift and rotate it open, then lift the globe out by its black handle and remove the waste drawer.

Check the seal strips on the inside front of the globe for peeling or trapped fur, since that can upset the sensors. Lean the unit back so you can see into the bezel holes, wipe each sensor lightly with a clean, dry cotton swab and clear the openings with compressed air. Put the globe back with the handle and waste port facing up and the large black gear meshed with the white drive gear, close the bonnet until the latches snap, plug in and wait for solid blue, then press Cycle.

## Wrong weights and missed visits

If the unit cycles fine but records the wrong cat or no cat, the cause is usually the pet profiles. Enter each cat's current weight in the Whisker app under Pets. SmartScale can take up to 7 days to learn, longer with several cats, and it cannot reliably tell cats apart when they weigh within 1 lb of each other. A cat that only stands on the step without going in may register a partial weight.

Firmware updates arrive over WiFi. To check, open the device in the app, tap the settings gear, then Firmware Information. Whisker says units with serial numbers starting LR4C can follow its manual update steps, and LR4S owners should contact Whisker Customer Experience.

## When to replace a part, call support or replace the robot

If the light still flashes red after cleaning, placement fixes and a firmware update, Whisker says the unit likely has a hardware problem. The parts are the Litter-Robot 4 Laser Board for the curtain sensors, or the Litter-Robot 4 Base when the scale itself is at fault. Red and green alternating means the factory scale calibration is lost, and Whisker says the base needs replacing.

Under the WhiskerCare warranty, log in at litter-robot.com, open My Robots and request the part; it ships at no cost. Out of warranty you buy the laser board, base or seal strips and follow Whisker's installation guide. Whisker support is at 1.877.250.7729, Monday to Friday 9 AM to 9 PM EST and weekends 9 AM to 5 PM EST. A new robot only makes sense if the base and laser board both fail out of warranty and the parts approach the cost of a replacement unit.

## Questions people ask

### Why does my Litter-Robot 4 think my cat is still inside?

Usually extra litter, a mat or cord under a foot, carpet, or the unit touching a wall. Remove litter above the MAX line, fix placement and press Reset twice at Home to zero the scale.

### Why is my Litter-Robot 4 not detecting my cat?

Cats under 3 lbs are not detected in Automatic Mode. Dusty curtain sensors and a scale that was not reset after moving or refilling also cause missed visits.

### How do I reset the Litter-Robot 4 cat sensor?

From the Home position, press Reset twice. The light bar flashes white as the scale zeroes. If it is stuck mid cycle, long press Reset for 3 seconds.

### What does flashing red mean on a Litter-Robot 4?

The cat sensor has detected a cat or weight for more than 30 minutes, and the unit will not cycle until the cause is cleared.

### How do I clean the Litter-Robot 4 sensors?

Wipe the bezel with a dry cloth, vacuum its inside edge with a brush attachment and use compressed air on the sensor holes. For a deep clean, remove the globe and use a dry cotton swab on each sensor.

## Sources

1. [Whisker: Litter-Robot 4 flashing red cat sensor fault](https://litter-robot.com/support/article/litter-robot-4-flashing-red-cat-sensor-fault)
2. [Whisker: Litter-Robot 4 inaccurate cat weight](https://www.litter-robot.com/support/article/litter-robot-4-inaccurate-cat-weight/)
3. [Whisker: Litter-Robot 4 laser sensor cleaning instructions](https://litter-robot.com/support/article/litter-robot-4-laser-sensor-cleaning-instructions)
4. [Whisker: Litter-Robot 4 Light Codes Guide](https://www.litter-robot.com/support/article/litter-robot-4-light-codes-guide)