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Bosch Machine F04 & Pump Runs Continuously


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Hello

I have a problem with a Bosch WFX3267GB.

All washes fail with error code F04 (blocked pump or blocked drain hose). The pump is clear, I have removed the sump (clean and clear) and checked the two-position pressure switch (contacts are OK measured by multimeter, I can make it switch by blowing into it and I hear it switching when I fill the drum via a jug of water).

At the beginning of any cycle the pump starts and drains any water within a few seconds. The pump then continues to run; after a few minutes F04 error is shown and the machine beeps: the pump continues to run until the power is switched off.

There seems to be no attempt to fill the machine: there's no sound from the inlet valves and as far as I can tell using the multimeter (it's tricky to connect the probes) there's no power applied to them. The valve's coils each measure ~4k ohm.

I'm not sure what to check next - any suggestions?

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Thinking about this, I can't see it being the pressure switch. The pump is controlled by the main computer chip. So, if as you say, everything is clear in the pipes, then I can only think of the board being at fault? The pressure switch would only switch things in and out of circuit when there is water in the drum, or it is empty. This doesn't switch in the pump.

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Thanks for the rapid reply.

To be honest, I do suspect the controller - it's just that it's very easy to blame the control board for something and then find out later on the problem was elsewhere.

I removed the top of the tube from the pressure switch and tried again after adding some water to the drum. It was exactly as before - the water drained but the pump kept running.

Is there anything else at all I should look at before attacking the control board?

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If the washing machine is giving an error implicating that it isn't pumping out the water, yet the water is pumping out okay, then the main suspect is the pressure system. Not so much the pressure switch, but a blockage in the hose leading from the pressure switch to the main tub or inside the pressure chamber bottle. Any blockage there will prevent the air pressure from being released in the pressure switch and the board needs the pressure switch to click off in order to tell it the water has gone.


Before delving into serious areas such as the main board, which can be very expensive, check this article which explains how the pressure system works - faults on pressure system

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Thanks.

There's no blockage in the hose - I stripped all of that assembly down when I removed the sump & made sure that I could blow through it & that there was no crud anywhere. I also checked as best I could that there weren't any leaks into it.

Wish it had been that - it would have been easy to fix...

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Do you know if you definitely have the correct explanation of the error code F04? I don't have the codes for WFX models but I have several other Bosch error code charts and the only one I have with F04 says it's a motor fault. However, that is for the WFP range. Unfortunately manufacturers often don't use the same error codes for all their machines, which makes life very difficult. I have other Bosch codes listed for other ranges which use just "4" and they do refer to timing out on empty but another Bosch model ranges use "4" to indicate "unexpected heating".

The ones that use the 4 to indicate timing out on empty (quoted at "6 minutes") all refer to potential causes as being blocked pressure switch, pressure switch fault, or faulty or blocked pump.

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For the hose: yes, it's clear all the way. I removed the sump, the pressure chamber and the hose. All are clean and clear. The pressure switch operates (as in I can hear it click + I've checked electrical continuity) both as water is added to the drum and then again as it is emptied.

For the error code, yes I'm certain it's the correct explanation. I'm reading from the original manual. The pump works fine and I checked the whole of the pressure switch assembly (several times...) before coming here to see if there was anything I'd missed. Also, the "6 minutes" sounds about right: on the cold cycle the timer counts down from 35 to 31 minutes before it throws the error message, but it seems like a long 4 minutes.

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Possibility is a connection problem between the pressure switch on the main PCB. That could explain why the pressure switch appears to be operating at the PCB is not receiving the signal.

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Does it work okay on spin?

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Also, do you mean you've checked for loose connections or you've checked the continuity of every wire on the pressure switch down to the board?

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I'm assuming the motor turns around on the spin cycle but it just doesn't kick into a proper spin? If so it definitely thinks it's full of water for some reason. After all the tests you've done it only leaves guessing at parts. If you guess at a faulty pressure switch and it's wrong it will be a hell of a lot cheaper than guessing at the pcb if it's wrong. If you replaced the pressure switch and it was no better though, that would really only leave the pcb..

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No, on the spin cycle the motor never starts, it just sits with the pump running. The only difference is that it doesn't eventually throw the error code.

The problem is that I'm 99.99% confident that the switch is OK, which only leaves the control board and the price for a new one of those will BER a 10 year old machine.

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Pressure switches can have multiple levels and can jam on one level. Therefore one could click up and down and appear ok but one level is jammed and it should click up twice and down twice for example - some even used to have tripple level switches with the top level acting as flood protection. That's how the older ones used to work. If you know how many levels it has (some were just 1 though) and can see that they are both working fair enough though.

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It's only a two-way and it's definitely not stuck. There's no significant contact resistance when it switches. I really can't see that trying a new one would help.

Which just leaves the board...

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Looks like it. Arh well you've tested as much as you can.

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Thanks for the update, that's good news. The main PCB is always the last thing we should suspect and try due to the cost, and the fact it genuinely is the least likely to be at fault most of the time. It's great if you are an engineer and have access to one you can quickly try but when having to buy one on a no return basis it's a big gamble.

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I still fail to see how it can possibly be the pressure switch. I thought the pressure switch simply made connections so the pcb could pass current through? However, why would the pump have anything to do with the pressure switch? Let's assume that the pressure switch switches the pump on and off. As water enters the drum, the pump is off. As soon as the pressure switch switches (due to being full of water) the pump switches on. If that happened no clothes would wash at all. The spin cycle gives it away, the pump is still running while the drum is empty of water.

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The fault described is, "error code F04 (blocked pump or blocked drain hose)". The PCB should only flag this error if it isn't receiving the signal from the pressure switch that all of the water has been pumped away. If the water hasn't pumped away, because of a blockage then the pressure switch will remain on and the PCB will check after a specific time and because the pressure switch is still on it will fail with "error code F04 (blocked pump or blocked drain hose)". On the other hand if the pump is working perfectly okay, and has successfully pumped away all of the water but there is a blockage in the pressure system, or a fault on the pressure switch, or a connection fault between the pressure switch on the PCB then it will also not receive the signal that the pressure switch has turned off and produce the exact same error.


The pressure switch is the only way the washing machine knows whether there is water in or not. The error code F04 is specifically saying that the main PCB has not received the signal from the pressure switch that the water has pumped away. It obviously assumes the pressure switch and pressure system is okay because it implicates a blocked pump or blocked drain hose but we know there is nothing blocked there because the water is pumping out perfectly okay.


Most washing machines these days set the pump running before filling up with water to start a new cycle. Before taking in any water at the beginning of the cycle the PCB will check that the pressure switch is turned off. If it is not turned off, and is still activated then it interprets this as being because there is already water inside the machine. It will therefore refuse to add any more water and trigger an error.


Many pressure systems also have a setting that directly energises the water pump. This is an attempt to deal with overfilling. If water ever gets inside the machine to such a level that it triggers the highest setting on the pressure switch this tells the PCB the washing machine is overfilling and it immediately energises the pump. It will keep energising the pump until the pressure switch turns off, but of course if there is a fault on the pressure switch or a blockage then it may not turn off and the main PCB will again assume the drum is full of water and act accordingly.


So the specific fault displayed by this washing machine was completely wrong. It highlights how unreliable error systems and error codes can be. There are few if any error codes that implicate the PCB. The PCB is the one thing that is controlling all of the error codes. In this particular case it appears to have turned out to be an erroneous error whereas the fault was inside the PCB but the PCB was implicating other parts. This doesn't happen too often, but it isn't that rare either. Which is one of the reasons I hate error codes.

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