First of all, as far as I can see there's no security issue here as it does not involve reprogramming through the OBD port or ECU connector and you've got to have access to the old ECU and the old ignition key. It's just moving the identity of the key and ECU from an old ECU to a new one. I am really struggling to understand why Mercedes / Bosch have blocked the replacement of the ECUs as they do fail sometimes and maybe it will have to be changed at some point in the life of the vehicle.

ECU failure is VERY rare, I've worked on a thousand cars over the years and only had to replace an ECU 2 or 3 times that I can remember. It really is quite unusual. In my case the glow plug light was always illuminated. I changed and tested all the other parts of the system and after all I decided I should be seeing 12v coming from the ECU to activate the glow plug relay but I was only getting 1.25v. None of this is documented anywhere so I was guessing.

The car I did this job on is a W211 2004 320 CDI with the OM648 engine. I imagine the fix would work on all Bosch ECUs of the era and for any make of car as no doubt they will have built the immobiliser the same way on all of them.

Alternatives to this method are to get some sort of decoding device off eBay which can plug into the ODB port and somehow manipulate the ECU into having no immobiliser. There are also companies out there who can reprogram your ECU to work around this issue. Lastly, of the methods I'm aware of you can replace 3 control units and the instrument cluster and use the matching key that goes with them. Get all the parts from another car but it is a big job transferring that many components and you would have to pay more for so many parts. All of these alternatives cost money and as with any attempt success isn't guaranteed. This method leaves your vehicle in standard factory spec and all you have to do is buy a replacement used ECU.

To do this job you must have a heat gun and a soldering iron which is small enough to work on tiny legs on modern PCBs. The tip should be no more than 1mm.

My original ECU and the replacement ECU have a manufacturing date which is only weeks apart and all the other numbers match. If you get your replacement used ECU from a car of the same type you shouldn't have any problem.

The ECU is a Bosch 0 281 011 698 and has the Mercedes part number A 648 150 01 79



To open the ECU you have to remove the torx screws on the back and prise open the cover. It is stuck to the PCB with glue down the flat side. It is also glued into the back cover.

This thread isn't really about opening the ECU but briefly, if you use your heat gun to soften the glue you can separate the ECU from the casing without damaging it. The technique I used was to wedge a flat blade screwdriver between the PCB and the case, apply gentle twisting pressure on it and heat at the same time near the screwdriver and wait until the force of the screwdriver and heat from the heatgun popped the ECU cover off. Inconveniently the immobiliser chip is on the reverse side so the whole circuit board has to be released from the casing.

Be careful doing this as you don't want to break the PCB or damage any of it. Practice opening the ECU on the failed one. Practice removing the chip from the new one. If you do it this way around you have one opportunity to fail once and your second attempt can be on the parts you must not damage!

Once the board is out, it looks like this. See the big grey splodges... that's where it is glued to the case.

The chip we want to move it the 8-legged chap sitting on its own at the top of the board. It is marked with the number 95320 - this number is common to a lot of Bosch ECUs of the era.



You have to use a heat gun to do the removal as trying to de-solder 8 tiny legs off the EEPROM is going to cause a headache. But this thread isn't a lesson in soldering so if you have your own way of doing things then feel free to do it your way.

Use tin foil to protect the surrounding area from direct heat from your heat gun. I cut a little glory hole in the foil which is 4 or 5 layers thick and you can see only the legs of the chip and not much else.



I don't know what sort of temperatures these things can withstand but I put a tiny flat blade screwdriver under the chip while I was heating it with the heat gun and when the solder was soft enough the chip broke free. You have to do this twice, once to remove your old chip and then a second time on the replacement ECU to remove the chip from that one. As suggested above, take the chip off the new board first as that's the one you're not using. Do not be too forceful with the pressure from the screwdriver as applying too much upward pressure can break the tracks off the PCB. You could also bend the legs on the chip. You have to find that sweet spot where the solder has melted but you've not burned a hole through to the workbench underneath!

There's nothing really more to add, it's not a step-by-step guide as this isn't a job for the faint hearted. Goes without saying if you mess this up somehow then you will have to go with one of the more expensive alternatives.

Solder your old chip (the right way around) onto the board of your new ECU, put the ECU back together and test it. Good luck.

If you wanted to be really clever you could reprogram the diesel injector trim values into your replacement ECU using a powerful diagnostics tool that has the capability. On mine the injectors are so old and dirty you can't read the trim values so it's not happening.

While this work around process for swapping in an ECU from a donor car can stand alone, it would seem to make sense to combine it with your glow plug issue thread so that members could see the rationale for even attempting this difficult task.

Unless the person attempting this has experience in electronic circuit board repair, I would predict the success rate to be near 0%.

And I’m actually surprised that the M95320 eeprom actually survived being brought up to the solder melting point (~280 F) to remove it from the original ECU as it has a rated operating temperature of 185 F.

The reason for not combining it with the glow plug thread is simply that a failed glow plug circuit is only one reason why you might have to replace the ECU. It has hundreds of functions and if any of them fail you would have to either fix the ECU or replace it. It is very rare that it has to be done... I've only seen a very small number of genuine cases for replacing ECUs in the past. What am I trying to tell people... I'm simply documenting the location of the chip and how to identify it. That's what people need to know if they are stuck with the same issue. I'm not a hugely experienced PCB technician so I'm not going to tell people how to solder as all I know about that work is what I've picked up over the years... I'm not trained in PCB work so have no business telling people how to solder.

In terms of testing the old ECU to see if it can be fixed... my only software I have access to is the MB Xentry dealer type stuff and it was responding to trying to clear the fault with a message saying 'pre-conditions have not been met' = it is saying the fault hasn't been fixed so it can't be cleared. I tried updating the ECU and that made no difference.

This wasn't a labour intensive job at all. It takes less than a minute to remove the ECU from the car and no tools are required for that. With the ECU on the bench it is less than 2 minutes to take the screws out. To get the glued covers off the PCB takes up to 5 minutes of carefully heating and prising. One of them clicked out in less than a minute and the other one took around 4 or 5 minutes to extract from the case.

In total it took about 30 minutes to swap the chip.

To add a bit of meat to the bone here... there is only so much testing you can perform on the glow plug system because it's quite a simple circuit. These are the things I did to eventually resolve the fault:

1. Changed the glow plug relay = aprox £30
2. Tested the resistance of the glow plugs, directly and through the wires
3. Checked the continuity of all the wires connected to the glow relay
4. Cleaned the ground point where the glow plug relay is earthed
5. Changed all the glow plugs for the purposes of elimination
6. Traced the other pin of the relay enliven circuit (green / white wire) back to ECU
7. Checked the voltage of the trigger wire coming from the ECU = only 1.25v - had to guess but felt this should be 12v
8. Concluded issue was the ECU
9. Bought a replacement ECU, plugged it in, car would not start but glow plug system WAS working, confirmed this a couple of times switching the new and old ECU in and out
10. Resolved the non-start issue with the new ECU by moving the immobiliser chip from my old one to my new one

I'm always open to learning so if you think you would have diagnosed it differently, what would you have done? Bearing in mind we have the benefit of hindsight at this point.

Congratulations on your success. The repair does presuppose a knowledge of what component on the PCB contains the component handshake ID, though. It also relies on a healthy background of general ECU operation and diagnosis. Component level trouble shooting is not for everyone, but for those with the patience and will, this is an example of a positive outcome.

This was the mode I was looking for when I put this up. I'm not going to walk people through how to solder... particularly as I'm not trained in it. YouTube is awash with guidance on this sort of thing. With my soldering skills I get by.

The key information is 'which chip' to change. Hopefully the photos and description are enough for anyone who has to replace an ECU and wants to move the chip. I got the general theme from YouTube but the only video I found on there is for a c-class and the ECU is a different version so the chip is in a different place.

I had to determine which chip was the right one as there is no information on doing this anywhere else for the W211 that I could find. I guessed that it would be a similar chip to the one in the c-class but there were quite a few 8-leg chips on the board so I narrowed it down by assuming it would be a lonely single chip sitting by itself. Many of the other 8-leg chips are sitting in pairs so this one I changed was isolated. It was a guess, but now that I've taken the gamble and changed that particular chip it is to the benefit of others as I've gone and documented it.

I thought it was good information and it would be worth a try. FWIW, at one time in my life I designed and bench tested digital circuits for instruments launched into Earth orbit so I found it interesting following the OP's chain of logic.

If you can get me a job launching satellites I'm willing to have a go.

Go to "swri.org" and "Careers". Salaried careers (no overtime) will be 4-8 year degrees and hourly careers (overtime allowed) will generally be 0-4-year degrees (NASA certs and other certifications are good to have). One of the best places in the world I worked. College campus atmosphere but a "not-for-profit" business with pressure to succeed. All profits are rolled back into new buildings, equipment, salaries, and internal research programs. So...if you want to get new lab equipment and a pay raise, you darn well better bring your project to the finish line within budget and on-time. Orbital dynamics are such that if one misses a launch window, it might be several years before you get another opportunity to launch. Therefore, smart and talented engineers and technicians who can solve problems and deliver on-time are well-rewarded. It was good enough I spent 25 years there...