iRobot Roomba Error Code 40: How to Fix It

Easy 5-20 minutes Medium Severity Verified June 2026
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Error Code
40
Brand
iRobot Roomba
Product Type
robot_vacuum
Severity
Medium
DIY Difficulty
Easy
Estimated Fix Time
5-20 minutes
iRobot Roomba Error Code 40 appears on Roomba Combo models when the robot detects that the water tank is empty or is not properly seated in its compartment. This error prevents the mopping function from operating to avoid running the mop pad dry and potentially damaging floors or the unit. In most cases, simply refilling and reinserting the water tank resolves the issue within minutes.
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Tools You'll Need

How to Fix Error Code 40

  1. Power Off the Roomba

    Never remove or reinsert the water tank while the Roomba is actively running or charging, as this can cause electrical contacts to arc or become damaged.

  2. Remove the Water Tank

  3. Inspect the Tank for Damage or Debris

  4. Clean the Water Tank and Contacts

    Do not use harsh chemical cleaners, bleach, or abrasive scrubbers on the tank or contacts, as these can damage the plastic and corrode the sensors.

  5. Refill the Water Tank

    Only use clean water or water mixed with iRobot-approved cleaning solution. Using other liquids such as vinegar, bleach, or scented cleaners can damage internal components and void your warranty.

  6. Reinsert the Water Tank

  7. Restart the Roomba and Test

  8. Reset the Roomba to Factory Settings

    Back up any preferred cleaning maps or schedules in the iRobot Home app before performing a factory reset, as this data cannot be recovered afterward.

  9. Check for Firmware Updates

Parts You May Need

Roomba Combo Water Tank
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Roomba Combo Mop Pad / Wet Mopping Pad
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Roomba Combo Water Tank
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Roomba Combo Mop Pad / Wet Mopping Pad
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Roomba Combo Water Tank
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Roomba Combo Mop Pad / Wet Mopping Pad
Check Price on Amazon
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When to Call a Professional

Contact iRobot Support or a certified repair technician if Error Code 40 continues to appear after you have refilled the tank, cleaned all contacts, reseated the tank firmly, and performed a factory reset with a firmware update. This may indicate a faulty water-level sensor, a damaged tank detection circuit, or a cracked tank that cannot hold water. If your Roomba Combo is still within its warranty period, do not attempt to open the robot's chassis — contact iRobot directly at support.irobot.com for a warranty replacement or repair.

Frequently Asked Questions

What does Error Code 40 mean on a Roomba?
Error Code 40 on a Roomba Combo means the robot has detected that the water tank is either empty or not properly installed. The mopping function will be disabled until the tank is refilled and correctly seated in its compartment.
Can I run my Roomba Combo without the water tank installed?
Yes, on most Roomba Combo models you can run the vacuum-only mode without the water tank installed or without attaching the wet mopping pad. However, if the mopping mode is enabled in your schedule or settings, the robot will trigger Error Code 40 when it attempts to mop. Make sure mopping mode is turned off in the iRobot Home app if you prefer to vacuum only.
What kind of water should I use in my Roomba Combo water tank?
iRobot recommends using clean, distilled, or filtered water in the Roomba Combo's water tank. Tap water can work but may leave mineral deposits (limescale) on internal components over time. Avoid using vinegar, bleach, essential oils, or any unapproved cleaning solutions, as these can damage the tank, the mop pad, and internal sensors.
Why does my Roomba keep saying the tank is empty even after I refilled it?
If your Roomba Combo still shows Error Code 40 after refilling, the most common causes are: the tank is not fully clicked into place, the electrical contacts on the tank or robot are dirty or corroded, the tank has a crack causing it to lose water, or there is a firmware bug. Clean the contacts with a dry cloth, ensure the tank is firmly seated, and check for firmware updates in the iRobot Home app.
How often should I refill the Roomba Combo water tank?
The water tank capacity varies by model but typically lasts one to two cleaning sessions covering an average-sized home (around 1,000–1,500 square feet). It is good practice to check and refill the tank before each mopping session to prevent Error Code 40 from interrupting a cleaning job mid-run.

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