How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask The Home Smithy Your Own Question
The Home Smithy
The Home Smithy, Home Appliance Technician
Category: Appliance
Satisfied Customers: 9622
Experience:  30 years repairing appliances
Type Your Appliance Question Here...
The Home Smithy is online now
A new question is answered every 9 seconds

Our Bosch SHU43C dishwasher has been growing black gunk on

This answer was rated:

Our Bosch SHU43C dishwasher has been 'growing' black gunk on the inside surfaces. We've drained, cleaned and replaced everything, and have checked the impeller blade and backwash plug - to no avail. Is the problem more likely with the Float or Float Switch? How do I check and fix if it is?
Hi. I'm Smitty. I am a professional appliance repair consultant here to help you diagnose, and repair, your appliance.
This sounds more like a mold problem than a mechanical one.
There is a product called Affresh. It is primarily for front load washers, but it is designed to prevent mold growth in the units also.
Run a tablet of Affresh through the unit (crumble it up and place it in dispenser cup), then run a plain water only cycle, and then use some Dishwasher Magic in a complete cycle after that.
Between the two products you wont have any more problems with the black gunk.
Note: you may have to run two treatments to get rid of it all. Mold can hide in the darndest places. If you see any hint of it do this again.
Please let me know if you have any other immediate concerns.
If not please select a positive rating at this time. Doing so will not close your question, and after you rate my service you may ask any followup questions you like. All at no additional charge.

Thank you.
Customer: replied 4 years ago.

We have tried Affresh several times and it does not change anything. There is no question in my mind that the problem is a water drainage issue, as there is standing water in the basin after nearly every (but not every) wash. A friend who is fairly knowledgeable about home appliances thinks that the float or float switch aren't working properly, but our manuals make no mention of either component.

If there is water left in the unit after the cycle is complete and the drain system is clear then you have an air gap issue.

What is happening is the water is being pumped out, and after the pump stops pushing the water, the remaining water in the first part of the drain hose slides back into the unit.

Installing an air gap will prevent this from happening. If an air gap is already installed you need to clean it out. Mold will plug them up (You should see what my moms looked like. YUCK!) and cause this problem.

Another thing that will help is leaving the door open a bit when the unit is not in use. The higher the moisture content inside the unit the more the mold will grow.

As for the float switch that is a possibility, but the float is not the only determining factor for stopping the pump.

Customer: replied 4 years ago.

I have no idea what "installing an air gap" means or how to do so, so your reply is not at all useful. Further, if a problem with the float/float switch is a possibility - which you acknowledge might be the case - guidance on how to access/clean/replace it would have been useful. I realize that $19 isn't much to pay for advice, but I was hoping to get some advice that would help me solve the problem, and so far it hasn't. I don't mean to be short, but this has frustrated us for a long time and we're tired of handwashing our dishes as we stand next to our non-working "top of the line" appliance.

All you have to do is ask! You dont have to leave bad marks on my record.
I have to go tow a friend off the highway right now. I will address your concerns as soon as I return.
Thank you.
Customer: replied 4 years ago.

You are right - I should have asked. Can I 'undo' the bad mark?

Stan, please accept my apologies for being a bit short with you. Please understand that the rating system is very frustrating to the experts here as it is used to decide who stays and who goes come time to 'trim the fat' so to speak. Unfortunately it is set up such that it is anything but an accurate way to tell who is and who isn't a good expert. It is geared more towards making the customer "happy" rather than providing the information they need to repair their appliance.
The only way to remove a bad rating is to select a ok or higher rating.

To access the floats that you are referring to you need to first remove the unit from under the counter. You may have to disconnect the water supply and electrical in order to pull it all the way out. That is dependent on whether the original installer left you enough slack to work with.
Remove the right side panel and you will see the float switches.
An "air gap" is an anti backwash device that keeps the siphon effect from drawing water back into the machine when the pumps impeller stops turning.
Here is a video that will explain all about a dishwashers air gap.
The air gap will not necessarily squirt water if it is plugged.

Customer: replied 4 years ago.

Thank you. I understand the frustration with the rating system, and again apologize for jumping the gun instead of asking for more info & clarification. I won't be able to do anything further until tomorrow, but will let you know how it goes! I assume that I will need to purchase the air gap and, if needed, the float switch from a hardware store, etc.; do you know who is most likely to carry them (i.e., Home Depot, Lowe's), or will I most likely have to buy from an on-line source? Thanks again.

Stan, the air gap is a common item available from any hardware store.

The float you will have to go to an appliance parts store to order. Bosch doesn't have any on line resources to order the part from.

Just look in your local phone book under appliance parts.

The Home Smithy and other Appliance Specialists are ready to help you
Customer: replied 4 years ago.

It took me a while to get back to this because none of the appliance stores in Savannah had an air gap. What I learned in the process, though, is that plumbing code here requires that the drain hose has a 'loop' in it between the appliance and the drain itself. Mine doesn't! The technician who advised me of this said that the loop functions in much the same way as the air gap, and will probably fix the problem. I'll be putting everything back together this afternoon, then will run it empty with an Affresh tablet. Fingers Crossed!


Thanks again.

For an air gap you should go to a hardware store.
As for there not being a loop in the drain hose Im surprised that you haven't had problems up until just now. That is part of every installation I do is the loop.
Let me know how it goes.
Customer: replied 4 years ago.

We've started using the dishwasher again, but will obviously have to give it some time to determine whether the problem returns. I now know enough to troubleshoot it again, but overall I'm just not very happy with the Bosch unit.


Thank you again for your help and follow up!



You are most welcome, Stan.
Have a great day! =)

Related Appliance Questions