Bosch Freezer unknown part - Page 1
Forum section: electronics / repair

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 27, 2025, 08:18:15 pm
content: Refrigerator model : Bosch KDN40A73 NO FROSTThe problem : Refrigerator running continuously creating huge amount of ice resulting high temperature to the fridge. Selected temperature -16C, measured temp -22CReadings : 1. Temp sensor (NTC) resistance inside the evaporator coils works as expected (5KOhm room temperature and about 25KOhm when temp is about -21C). 2. Safety fuse has continuity 3. Defrost heater resistance about 250ohm and 4. Evaporator fan about 500ohm.All of them seems normal, fan works and defrost also works (I hear the ice cracking)(?) Unknown part and it's role: The white sensor in the back of the freezer. It is connected directly to the logic board. The resistance reading just after the fridge it was shutdown was about 4.5KOhm. After an hour with fridge still powered off, is stabilized to 5.3KOhm. When I keep the sensor with my hand it seems the reading is dropping down to about 3KOhm. So, the question here what exactly is that part and what it's role to the system? It seems like a temp sensor but the freezer has already a an NTC sensor inside the evaporator coils. The measurements are considered as normal? If its a failing part how can I find a replacement?

author: indeterminate
author_rank: Regular Contributor
timestamp: September 27, 2025, 09:36:13 pm
content: Is that a mechanical temperature sensor on the wright hand side.it is normal to have a sensor for the compartment and one for the defrost circuit

author: abrasumente
author_rank: Contributor
timestamp: September 27, 2025, 11:34:18 pm
content: Quote from: indeterminate on September 27, 2025, 09:36:13 pmCan confirm. I'm an HVAC/R tech and this is typically how it's done. Does this fridge have digital controls? Also might consider if the start relay is stuck, keeping the compressor active. They're mounted directly to the compressor under a little black cover. Do you hear the evap fan shut off but the compressor keep running? That'll cause ice in a hurry.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 28, 2025, 05:57:20 am
content: Quote from: indeterminate on September 27, 2025, 09:36:13 pmSo it's a normal temp sensor just for the freezer. The measurements I have done seems normal to you? I expected to have a measurement above 20KOhm just after the freezer was shutdown. Despite that the reading was about 5KOhm.How to find the exact part number for that? Is strange that is direct connected to the logic board without connector. Is that because Bosch believes that this part it will never go bad?

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 28, 2025, 07:25:39 am
content: Quote from: abrasumente on September 27, 2025, 11:34:18 pmYes it has digital controls.I am pretty sure that a) when the evaporator fan shut off i.e open the freezer door the compressor, is keep running and I am totally sure that b) when evaporator heater is active, i.e no frost mechanism is heating, the compressor also keep running.If the relay is stuck, can be repaired or should be replaced?

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 28, 2025, 07:29:24 am
content: Quote from: indeterminate on September 27, 2025, 09:36:13 pmThe mentioned temperature sensor is on the left side (and not at right you wrote) at the attached photos, just above of the multiple connectors below.What compartment sensor offers in the whole system at the moment we have already a sensor inside the evap coils?

author: abrasumente
author_rank: Contributor
timestamp: September 28, 2025, 12:36:16 pm
content: 10k ntc temp sensors are most common in comsumer goods but I've seen everything from 1.8k to 100k. If its a 10k, its WAY off when holding in your hand. If you dont have a service manual, grab an accurate thermometer and a glass of ice water. Dunk it and the sensor into the glass and read the resistance after several minutes. Check against freely available online tables and see if the resistance corresponds to the temp, it doesn't have to be exact. In your hand this reads like a 5k ntc, so dropping it in cold water should make the resistance go up quickly into the 9-10k range. If it doesn't move much I would replace this sensor.If it has digital controls, it's likely there's a diagnostic mode. The crux will be what combination of keypresses gets us into it.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 29, 2025, 04:51:06 am
content: If you could provide us with the complete model number, we will be able to find the correct spare part list and sensor.For model KDN40A73, Bosch lists variants with extensions from /1 up to /13, and for KDN40A73NE from /1 up to /7. Therefore, only the model number without its suffix is insufficient.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 29, 2025, 07:48:04 am
content: Quote from: Uunoctium on September 29, 2025, 04:51:06 amYes, of course. The model isKDN40A73/04.The measurements were all good.1. The start relay has a resistance between Common - Start connections of about ~30Ohms and OL between Common - Run (Off). Didn't test when the compressor is running, but it starts just fine.2. Relay protection has continuity3. Replaced the temperature evaporator sensor with the original one. Testing resistance with a glass of water and ice cubes seems to respond from 5KOhm (room temperature) to about 13KOhm in 5 minutes4. The white compartment sensor -or unknown part for me in the beginning- in the back confused me. At room temperature, the resistance was about 4.4kOhm, and testing with glass with ice cubes, I had no more than 8.6kOhm. It seems a little low to me.I ordered a new one, without being sure if it is the correct part.https://www.mrgranazis.gr/shop/eshop/antallaktika/oikiakes-syskeyes/psygeia/aisthitires-thermika/050626021-aisthitiras-psugeiou-5-kohm-tupou-bosch-00619691/?v=8cf4d375016a5. The safety fuse had continuity7. Evaporator fan resistance ~504Ohm. It stops working when the doors are open,BUT the compressor keeps running.8. Defrosting Heater resistance ~284Ohm9. Logic Board model 9000-451-518, EPK 64870 (attached photo), seems to be in good condition. No bad capacitors or burned parts.I powered on the refrigerator. Currently, it works as expected, but the problem appears after 10-15 days when the fridge compartment increases progressively its temperature.I noticed (check the photo) that the ice was mostly in the front part, blocking the cool air from flowing into the fridge.The rubbers are all in good shape. I also added Vaseline to soften them. With a light test inside the freezer, no leaks were found.It drives me crazy, this refrigerator..... I doubt that Bosch Refrigerators are reliable devices at all.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 29, 2025, 08:48:49 am
content: If it's the NTC and i throw "Bosch + NTC + Fühler + 00420667" in search engine, on page 3 or 4 alternate offers for aftermarket parts or clones occur. Most of beginning with "for Bosch" "as Bosch xy" but often at half the price.eg:https://www.ersatzteilshop.de/temperaturfuehler-ntc-passend-fuer-bosch-00420667-bluparts-in.htmlEdit:Here comes the Bosch Greek version of your sparepartshttps://www.bosch-home.gr/supportdetail/product/KDN40A73NE/04#/Tabs=section-spareparts/

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 29, 2025, 01:28:16 pm
content: Quote from: Uunoctium on September 29, 2025, 08:48:49 amThis part is referring to the evaporator temperature sensor but the part in question is the compartment sensor, with white cover (the one you can see with the multimeter measurement)Regards

author: Haenk
author_rank: Super Contributor
timestamp: September 30, 2025, 06:38:17 am
content: Maybe hunting a red herring here?As the sensor has changing values, I'd consider that "probably working OK" as of now. However I would not trust the PSU unit - running 24/7 for a decade or even longer might dry out even quality caps, I would check the supply voltages (stability and line noise) and maybe even replace the caps for good measure. Line noise or wonky voltages can cause all kinds of issues with the microcontroller...Another failure is of course the relais, which might get stuck. They are not horrible expensive (a couple of EURO each) and rather easy to replace.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 30, 2025, 09:15:37 am
content: Quote from: Haenk on September 30, 2025, 06:38:17 amHi, thanks for the reply.I replaced the logic board in 2021. Back then, for some reason, the board didn't signal the compressor to power on sometime after several days of stable operation. Very odd !!So, yes, I can't rule out the possibility that it has a problem again. I have had negative experiences with noise suppressors and microcontrollers in the past. I understand exactly what you mean.For now, it works as expected, but it's too soon for conclusions. I will wait a few days to check the working state. If the problem continues, I will inspect and replace relays & all electrolytic and polypropylene film capacitors. If I find nothing, I will replace the logic board again.It's very frustrating when the problem occurs again and again with the board. Many refrigerators last for years, but this particular one is disappointing. It's 15 years old and has accumulated many issues over time.

author: sdancer75
author_rank: Frequent Contributor
timestamp: October 10, 2025, 05:21:48 am
content: Hi again, I am back to give feedback on the mentioned problem. Good news.After almost 15 days of work, the problem reappeared. So yesterday, I put off the logic board. I desoldered all electrolytic & polypolyphenylene (noise suppression) capacitors.The electrolytic was found to be good, and soldered again. However, the X2 polyplopylene capacitor was found to be out of specs. The nominal value is 100nF, but was found to be about 87nF. I replaced it. I cleaned the contacts, then set it to work.Now it seems to work as expected, but I have to wait for a while to be totally sure. The evidence until now are: a) the fridge reached the appropriate temperature, b) the compressor does not run all the time, and c) the water tank has water for the defrost cycle.PS: I also had the chance to check the old logic board that was replaced back in 2021. I found one relay to be bad (the one that gives a compressor signal to start/stop). The coil was good, but the inner contact was broken.