Direct answer
For the Bosch 300 Series WGA12400UC washer, water entering an idle washer points to an inlet valve that is not sealing or drain-water backflow. First, close both supply faucets and observe whether the water level stops rising.
What this covers
- Bosch 300 Series WGA12400UC washer (WGA12400UC)
- Bosch compact front-load washer; confirm the North American model and any paired condensation/heat-pump dryer installation.
- The specific fault this record addresses: fills with water while turned off
What it does not cover
- ×The same symptom on a different Bosch product line, which has its own record and its own cited source
- ×An on-screen error code whose meaning has not been checked against this exact model's manual
- ×Live electrical, gas, sealed-system, internal battery-cell or structural repair — a technician boundary, not a self-service step
Differential
What actually causes this, and how to tell them apart
These are ordered by how often each one is the answer, not by how easy it is to try. The point of the middle column is that two causes producing the same headline symptom rarely produce the same detail — that detail is what tells you which one you have before you change anything.
- 1
An inlet valve is not sealing fully
What separates it Clean water accumulates while the machine is off, and stops rising once the supply faucets are closed. Sediment or debris holding a valve seat open is the usual cause.
How to confirm Close both faucets. If the level stops rising, the water is coming from the supply side and the valve is at fault.
- 2
Drain water is siphoning or backing up into the tub
What separates it The water is dirty or soapy, and continues to appear with the faucets closed. Either the drain hose is inserted too far into the standpipe, creating a siphon, or the household drain is backing up.
How to confirm With the faucets closed and water still appearing, the source is the drain — check the hose insertion depth and standpipe height.
- 3
The drain hose has no high loop
What separates it Water drains out of the tub between cycles, or drain water flows back in, on a machine whose drain hose runs flat or downhill from the outlet.
How to confirm The drain hose needs the high loop or standpipe height the manual specifies to prevent both siphoning out and backflow in.
Before you touch a setting
Record this first — changing settings destroys the evidence
Every item below stops existing the moment a reset, re-pair or settings change is made. Written down first, they are what separates the causes above; recovered afterwards, they are guesswork.
- Whether the water is clean or dirty and soapy — this identifies supply-side versus drain-side immediately.
- Whether the level stops rising once both supply faucets are closed.
- The drain hose routing, insertion depth and standpipe height.
Close both supply faucets immediately
ReversibleThis is both the diagnostic and the containment step — an unattended fill can flood a room.
Observe whether the water level stops rising
ReversibleStopping means a supply-side inlet valve; continuing means drain-side backflow.
Check the drain hose insertion depth and required high loop
ReversibleA hose pushed too far into the standpipe siphons; one with no high loop allows backflow.
Keep the faucets closed until the inlet valve is replaced or the drain is corrected
ReversibleLeaving the supply open to a valve that does not seal risks flooding while unattended.
Monitor the empty tub with water restored after the repair
ReversibleConfirms the fix before leaving the machine unattended again.
Model-specific
What is true of this model and not of its siblings
Generic advice for the product line fails here. Each item below is a property of this specific model family that changes which of the steps above apply to you.
- Model scope
- WGA12400UC. Bosch compact front-load washer; confirm the North American model and any paired condensation/heat-pump dryer installation.
- Type — COMPACT 24-inch FRONT-LOAD
- A 2.2 cu. ft. COMPACT 24-inch FRONT-LOAD washer, stackable with a compact Bosch dryer. Small drum, small loads — overloading causes poor results and unbalance.
- AquaStop leak protection
- AquaStop shuts off the water if it detects a leak; a water/tap error with water in the base is AquaStop tripping — find the leak rather than overriding it.
- Quiet & sensing
- Quiet brushless motor and ActiveWater Plus automatic load-size water setting (low water level is normal); a SuperQuick 15 cycle for small loads.
- Size class
- One of the smaller models in its line, which has a direct consequence: a smaller body holds a smaller battery, so its rated runtime is genuinely lower than its larger siblings. Runtime that looks poor beside a Max is usually this rather than a fault.
- Symptom boundary
- This record covers Bosch 300 Series WGA12400UC washer when it fills with water while turned off. A different symptom on the same hardware, or this symptom on a different Bosch product line, has its own record with its own causes and its own cited source.
Stop boundary
When this stops being a self-service repair
Continuing past any one of these costs more than the repair saves — in safety, in warranty, or in evidence a technician needs.
- !Close both faucets immediately and leave them closed if the machine fills unattended — this symptom can flood a room and is one of the few washer faults that causes major property damage.
- !Dirty water backing into the tub with the faucets closed indicates a household drain problem, which needs a plumber rather than an appliance technician.
Evidence ledger
Sources behind this answer
Visible sources support the visible claims. Home Product Support records the publisher, the fact used and the review date; community reports can suggest an issue but do not become a published fact on their own.
Where the manufacturer's article stops. Bosch Home Appliances publishes this as "Owner support". Official Bosch appliance manuals, error-code help, cleaning and service entry point. What that article does not carry is the model boundary — it is written for a product line, and WGA12400UC is the scope applied here. Where a control label, terminal, indicator pattern or reset sequence differs on your unit, the manual for that exact model is the authority, not this page and not the article.
Exact questions
Common follow-ups
My compact Bosch WGA12400 vibrates or 'walks' during spin.+
Compact 24-inch washers vibrate more than full-size, especially the high spin speeds Bosch uses to extract more water. Confirm the shipping bolts were removed at install, the feet are level and locked on a solid floor, and — if stacked — the stacking kit is fitted. Keep loads modest and balanced, since a small drum unbalances easily. Those installation checks resolve most compact-washer vibration.
Water keeps appearing in the drum when the machine isn't running.+
Close both supply faucets right away, then watch whether the level keeps rising. If it stops, an inlet valve is not sealing and the faucets should stay closed until it is replaced. If it keeps rising with the faucets closed, the water is coming back up the drain hose instead.
Does this answer apply to every Bosch model?+
No. It is scoped to WGA12400UC. Bosch compact front-load washer; confirm the North American model and any paired condensation/heat-pump dryer installation.
What should I record before troubleshooting fills when off or water appears or inlet valve?+
Record the complete model identifier, exact message or indicator, software/firmware where visible, the operating stage and what still works.
Why is a factory reset not the first step?+
A reset can erase accounts, networks, maps, schedules or preferences without distinguishing fills when off, water appears, inlet valve. The ordered checks preserve that evidence.
When should I stop and use qualified service?+
Close the water immediately for continuous fill or overflow risk.
Answer for the exact device and software version shown above.