My last hope - trying to trick a fluid sensor into thinking it's full - Page 1
Forum section: electronics / repair

author: hhoarder
author_rank: Newbie
timestamp: April 07, 2025, 12:06:28 pm
content: My car has a failed/faulty fluid level sensor (for AdBlue) and I only have a few dozen km left before it won't drive anymore. Believe me when I say I've tried every trick into resetting the system, but nothing works. My last, desperate attempt is somehow sending a "tank full" signal back to the ECU. A replacement tank is only deliverable in July, maybe even later, which basically turns my car into a brick until then.The entire module is Bosch Denoxtronic 5, which contains a heater element and an ultrasonic level sensor. The heater works fine. This is an integrated unit inside the tank and you cannot replace any part of it, just the entire tank.Here is the wiring diagram for the level sensor part:As you can see, there are 3 wires; positive, negative and signal. My hope is to somehow send a "tank full" status back to the car via the signal wire. There aren't any specs that I could find explaining what type of protocol is being used. The control module itself is reporting raw values in the range of 0mm to 245mm.I've disconnected the plug from the sensor today and I've measured two things inside the plug; voltage across + and - is 12V as expected for a car. However, there's also 7.2V between + and signal, which I didn't expect.I suspect this means the car is trying to establish some sort of communication by sending 7.2V to the sensor? I was hoping the signal wire would be a one-way connection and I could simply generate voltage on it to report tank full.I do have Arduino and Raspberry if I'd need to generate a PWM. I've also read that some cars use LIN for transmitting messages and there seem to be some Arduino libraries for that as well. Sadly, there are no specs to help figure out what exactly is being used.I am very grateful for any tips or tricks you might offer that would help me resolve this, thank you!

author: Swake
author_rank: Frequent Contributor
timestamp: April 07, 2025, 07:33:12 pm
content: Could you post some pictures of the sensor please. And all information you have about any markings on it.What is the car brand/model/year? Do you have the exact engine type, I mean the one used by the manufacturer?Careful, more and more car sensors are now 5 Volt.

author: Swake
author_rank: Frequent Contributor
timestamp: April 07, 2025, 08:13:23 pm
content: Something from BMW. See page 1 for a description of the signal.I have no idea if this applies to your situation.Note that you could also perform an Ad-blue Delete. You'll have to contact a 'tuning' garage for this reprogramming. I don't know how legal this is in your case or even if you would still make it trough the annual technical check for the car. (MOT in the UK)

author: amyk
author_rank: Super Contributor
timestamp: April 07, 2025, 09:50:16 pm
content: The document you linked has a funny message:"Engine start without Exhaust Fluid/AdBlue no longer possible. Exhaust Fluid/AdBlue must be refilled.Driveto the nearest BMW Service Department."Do you have an oscilloscope? I'd check what the signal line shows.

author: hhoarder
author_rank: Newbie
timestamp: April 08, 2025, 07:47:58 am
content: Quote from: Swake on April 07, 2025, 07:33:12 pmSadly no, the sensor module is built into the reservoir itself and cannot be accessed or removed individually. You can only replace the entire reservoir with all the electronics inside. The most info I could find out is that it's a Bosch Denoxtronic 5, but they have no specs that I could find anywhere, just some promo material.It's Mercedes V250, year 2014. Don't know the exact manufacturer engine type, but the same reservoir is used in multiple of their models.

author: hhoarder
author_rank: Newbie
timestamp: April 08, 2025, 07:50:38 am
content: Quote from: Swake on April 07, 2025, 08:13:23 pmThat was my thought as well, but it's illegal here in Germany as well. You can still get it done, but like you've said, no idea what would happen at the technical check then. I can only assume they will get more thorough with time as more and more people are having EDF removed.

author: hhoarder
author_rank: Newbie
timestamp: April 08, 2025, 07:51:42 am
content: Quote from: amyk on April 07, 2025, 09:50:16 pmNot anymore, I actually sold my mid-tier scope a few months ago since I haven't used it for years. Who'd have thoughtWorst case, I could buy cheap ones that connect to the phone, should be good enough just to see what kind of signal it is.

author: Swake
author_rank: Frequent Contributor
timestamp: April 08, 2025, 08:32:04 am
content: If the supply module with the sensor is 'glued' onto the tank this means you have a second generation Bosch Denox. Unless a trained individual using the specific Bosch diagnostic tools analysed the situation at this point you don't know if it is this sender module in the tank that is dead or if the controller that it is connected to died. Checking the OBD error codes, if any, is not enough, even the in-house Mercedes one is not going to tell you what you need. The usual automotive parts-replace-cannon problem.The first generation of these adblue sensor/pump/heater combinations are notorious to fail, but it is not the sensors that fail. The controllers die. Unfortunately, I don't have information about the second generation.Without exact information and you not having a working sample to measure and compare, it is near crystal ball voodoo level stuff trying to help you further.I suggest you to check the second hand market. With the exact right model number that must be stamped somewhere on the tank you'll certainly find one.

author: hhoarder
author_rank: Newbie
timestamp: April 08, 2025, 05:57:15 pm
content: Quote from: Swake on April 08, 2025, 08:32:04 amIt's actually Denox 5, not 2. I've used a camera to check inside and looking at photos online, it's definitely 5 as it has a white mesh filter, which the 2 doesn't.I was able to connect to the controller and run various functions and tests on it, so I'm assuming it's working. The same controller is also responsible for the heater element and the pump, both of which I could turn on and verify they're working. Of course, the controller might have failed only partially, but that seems unlikely if everything else is still working. Unless you mean the controller inside the tank that does the actual ultrasonic measuring, in which case you're spot on, near impossible to test for that.I've actually ordered a used tank just a few hours ago. The seller has a 2-week return policy, so either it fixes my issue or I haven't lost any money. Trying to read LIN messages (if it's even LIN) and then sending a fake signal back might take me days or weeks of worth and I've already wasted a ton of time trying to fix this.In any case, thank you very much for your help and replies, I sincerely appreciate your time and effort!

author: Swake
author_rank: Frequent Contributor
timestamp: April 08, 2025, 06:14:33 pm
content: Denox 2 is the first generation. Denox 5 is the second generation. Go figure, marketing boys stuff....Good luck with the used tank. Let us know if it works !

author: coromonadalix
author_rank: Super Contributor
timestamp: April 09, 2025, 08:26:23 am
content: you should search the "diesel delete" on some cars forums MHH and otherssome answers partials or not-Disconnect Dpf Pressure sensors and both Dpf Temp sensors-Disconnect Adblue Ecu

author: Haenk
author_rank: Super Contributor
timestamp: April 09, 2025, 08:49:09 am
content: Quote from: coromonadalix on April 09, 2025, 08:26:23 amThat won't help a lot in Germany. German TÜV (that's like british MOT) has to be done every two years, including emissions check. And they are quite nitpicky. A failed number plate bulb? -> You are OUT. (Yes, that has happened to me.)The illegal modification is only a temporary solution. Sidenote: Truck drivers like the Adblue delete a lot, because it saves money - german roadway police is very aware of it and does check at times - they have a very good idea of what to check and where to check, no hiding there.

author: hhoarder
author_rank: Newbie
timestamp: April 09, 2025, 09:09:20 am
content: Quote from: Swake on April 08, 2025, 06:14:33 pmAh, sorry, I didn't know that! That makes no senseThe tank is arriving tomorrow, hopefully that solves it. I still can't imagine how they think it's normal having to wait 3 months for a spare part that otherwise renders your car useless.

author: hhoarder
author_rank: Newbie
timestamp: April 10, 2025, 06:02:59 pm
content: Happy update - I've replaced the old tank with a used eBay one and the error went away in 10 seconds after filling it. So it was definitely a failed level sensor that was causing the issue. Hopefully the used one will last at least a few more years as I'm sure this will eventually come back. At least I know how to replace it nowThank you to everyone who's tried to help me with various ideas!

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: April 10, 2025, 06:17:18 pm
content: Quote from: hhoarder on April 07, 2025, 12:06:28 pmYou can use a logic analyzer or oscilloscope to record the waveform on the signal wire (pin 5). Determine if it matches LIN protocol or is analog/PWM. If you become sure that it supports LIN protocol, based on that you can build a LIN-spoofing Arduino setup. In case it is PWM, try faking a signal with PWM/analog voltage.Monitor the sensor value via OBD2 while testing. You may find the following links helpful.https://hackaday.com/tag/linbus/If you want to know how to test a coolant temperature sensor, you can check this out.https://www.ancel.com/blogs/news/test-coolant-temperature-sensor