Sounds frustrating. I dont know the answer but your post in Chat GPT gives the below response. It may be useless, or one of the points may help, i dont know.
AI:
Here are some less obvious but technically plausible causes that can create persistent foaming in a Siemens dishwasher, even with no detergent or rinse aid. You’ve clearly done all the standard checks, so I’ll focus on deeper, engineering-level possibilities that fit your symptoms.
1.
If old detergent or surfactant has become trapped in a location that still contacts water during the cycle, it can keep producing foam long after you stop adding detergent. This is surprisingly common when:
• The sump has biofilm that interacts with soft water and aeration
• A detergent overdose months ago coated internal plastic surfaces
• Surfactant has migrated into hoses or the Zeolite ducting and is being reactivated by hot steam during drying
Test:
Run the machine completely empty, then stop it after 10 to 15 minutes. Scoop out and examine the water in the base. If it
already shows suds early in the cycle, the contamination is being released at the start, not generated at the end.
Potential fix:
Try running three back-to-back long cycles with
a full kettle of hot water poured directly into the sump at the start to dilute any residues, plus
no tablet, possibly with a
cup of white vinegar placed upright in the top rack. Hotter water in the sump helps lift residues that normal modern eco cycles won’t.
2.
A stuck regeneration valve or incorrect salt dosing can result in
over-softened water, which massively increases foaming due to lower surface tension. With soft water, even tiny residues can foam like mad.
Checks:
• Set the machine’s water hardness to
maximum (the highest dH setting), which reduces the amount of softening performed by the internal softener.
• Temporarily
remove all salt and run multiple cycles. Over-softening can happen even when the tank appears clean and the brine seal is fine.
If the foaming reduces when hardness is set higher, the softener is overdosing.
3.
Some Siemens dispensers can fail so that a small amount of rinse aid leaks during each wash, regardless of the setting. Since rinse aid is a surfactant, that alone can create significant foam.
Test:
Completely empty the rinse aid reservoir with a syringe or turkey baster and leave it dry. Wipe the dispenser well.
If foam disappears after 3 to 6 cycles, the dispenser is leaking.
4.
The Zeolite drying system uses very hot, dry air. If even a small surfactant residue is present in the sump, the sudden injection of very hot air near the end can cause violent foaming. The Zeolite doesn’t create foam chemically, but it can cause agitation and temperature shifts that reveal foam.
Test:
Run a short wash programme that
does not activate Zeolite drying. On many Siemens models, this will be the 45 C quick wash or pre-rinse.
If there is no foam on quick wash but lots on auto or eco, the Zeolite airflow is stirring something that shouldn’t be there.
5.
This is rare but does happen. Examples:
• Silicones or surfactants from hand wash, washing up liquid, or shampoo splashes on dishes
• Dishwasher being used to wash things like reusable bottles with soap trapped under caps
• Using sponges or cloths washed in laundry detergent to pre-clean dishes
Since you’ve been troubleshooting for months, this might already be ruled out, but it’s worth considering.
Test:
Wash an entire load of
brand new glassware and ceramic plates straight from the cupboard with nothing added. If foam still appears, external contamination is ruled out.
6.
If the drain hose or non-return valve is not sealing perfectly, the machine can pull back a small amount of soapy water from the sink trap during certain parts of the cycle.
Even a tiny amount of washing up liquid entering the sump will produce enormous foam.
Checks:
• Ensure the drain loop is at least 40 cm above the bottom of the dishwasher
• Check the sink trap for any signs of dishwasher water entering it
• Remove and clean the non-return valve in the sump (easy and quick on Siemens models)
7.
If the NTC temperature sensor has drifted out of calibration, the heater may spike, superheating small amounts of surfactant residue and causing foaming. This may not be detected by the engineer unless the fault is intermittent.
Signs:
• Steam seems excessive
• Plastics come out hotter than normal
• Foam appears suddenly, not gradually
A software upgrade is unlikely, and Siemens are telling the truth there. But sensor drift is absolutely possible on a seven-year-old machine.
If you want the most effective path forward
I’d try the following in this order:
- Completely empty the rinse aid reservoir.
- Set the machine’s water hardness to maximum (reduces softening).
- Remove salt entirely for a few days.
- Run two very hot intensive cycles with a kettle of boiling water added each time.
- Run a short cycle that doesn’t use Zeolite drying.
- Check the drain loop and non-return valve.
If you want to, I can give you a step by step diagnostic workflow that narrows it down conclusively in fewer than six cycles.
