$36K is indeed quite hefty for a small shop.

Do you have any pictures from inside the module? some of the pictures load but those ones don't.

Recently changed the plugs on my 2020 I6 and I was aware of the potential issue so I was extra cautious. The 2020 harness didn’t show any signs of deterioration, but I still handled it carefully. I did not have any issues other than removing plug #6.

I have replaced them on my 2020 gls450 at 60kmi. Not a complicated job except some dissasembly of the air intake pipes to acces the last couple plugs. Compared to my 2013 GL450 8cyl it is a much easier procedure. Just remove wires carefully and lubricate with a dielectric grease upon reassembly. Also, orient the plug electrodes properly.

Could be, but that is not necessary if you detach the wire carefully from the plug.

Unless the harness has deteriorated. Which would be covered if still under warranty.
So before you're out of warranty, have the dealership inspect.

I am considering buying a 3rd generation 2019 to 2023 CLS 450 with the M 256 engine. A lot of the available cars have less than 60,000 miles on them and the spark plug replacement hasn't come up yet. How prevalent is this problem? Did the wiring harness issue ever get resolved? Any advice will be welomed.

So I had indie replace my spark plugs at 40k. And then this problem came up at 74k at the dealer.

Thanks for posting.
Two things are interesting about your experience.

One - a spark plug harness failure can activate a check engine light.
Two - that an "extended warranty" can cover it.

A couple of questions, though - was this Extended Warranty one from the factory, specifically on the harness? Or was this an Extended Warranty that you purchased, like a CPO or factory ELW, or a Service Policy like Fidelity Platinum?

Dealers are funny.

I did an oil change at around 50K miles, and dealer wanted me to also do the spark plugs. I declined and simply told him, nah thanks, am selling the car in a year and likely won't even hit the 60-65K. It's not worth it to do spark plugs for the next owner. He offered 20% discount on their 1200 USD estimate, and I declined again.

Later they called me, and said: Hey we will also replace your wiring harness (which will likely fail later when car is out of warranty) at no charge to you now (which is usually a 3500 USD job that warranty will pay us for). I said HOW? there is no check engine light, nothing. He said: Well, we are 100% sure that your wiring harness will break or will need to be replaced and can get the warranty to pay for our labor and the new harness, if you let us do the spark plug job (which I will pay for).

So I was like: YOU WANT ME TO FIND YOU A CAUSE TO GET PAID 3500 USD, when nothing is wrong today with my wiring harness, and the key to do is for me to accept and pay 1200 USD for a spark plug job that I don't feel I must do now. How about we split the 3500 USD in half, and you do spark plug for free, yet you would get the 3500 USD for the wiring harness (which according to him, he is confident he will get it covered by warranty, even without seeing it). To me, this feels like a scam. Dealers are scamming Mbenz, or maybe the wiring harness is just that bad that as soon as you touch it for a spark plug job, and it wall fall apart.

Well, I'd rather not worry about all this nonsense, and just drive my car to 60K miles or 65K miles. Am gonna sell it by then. Me doing spark plugs now means only this: 1. paying it for the next owner; 2. finding a cause for the dealer to get paid 3500 USD before my car is out of warranty. Am not getting any benefit unless am truly going to keep the car for 2-3 more years or many more miles. Remember folks: Dealers LOVE to find something wrong with your car and get it covered by a warranty claim to be paid by corporate

Thanks for your reply. I'm a big 2nd generation CLS fan and hated when the model line changed and then was discontimued with the 3rd generation. Perhaps it is best for me to steer clear of the 3rd generation CLS 450 models that have not corrected the wiring harness problem. Such beautiful cars, too bad they are not built to last past a reasonable period of time or mileage without something major happening.

Preventative maintenance.
You can choose to do it or not, but in this case you might end up needing it unexpectedly, so why not take advantage of it?

It's a known issue. I'd be pissed if they didn't tell me about it.

My dealer did tell me about it at about 45,000 mi, they fixed it, and now I don't have to worry about my check engine light coming on and having to schedule another appointment.
It saved me time and money, and a bit of worry.

Cause they want to charge me 1200 USD for something I wouldn’t need. They want me to be the one asking for a spark plug change job just so they can then squeeze in the harness warranty job and get paid 3500 usd by warranty in addition to my 1200 usd. I don’t need sparks as we will be trading in the car maybe next year and likely before 60-65k miles.

Just ask them to change your wires.

They claim that they “need” a reason to look at it first (like hey we replaced the plugs and then they got damaged or whatever).

Thanks for confirming that the engine wiring is a known problem. I'm sure there is a way to take care of this and still enjoy these cars.

W_222 I would be one of the buyers who would be in the market for a CLS like yours. If you decide to sell yours, let me know.

That's not logical or acceptable.

They could easily say "Hey we looked under the hood and it needs new wires!"
I'd probably stand there and raise a stink.
After asking them if they could look under the hood to see if it needs new wires.

On repair order: "Customer states that he needs new plug wires."

But you are just confirming my point. My harness is good. They just THINK they can simply authorize and claim that the harness needs to be replaced just so they get paid for a big job by warranty, during the process of replacing the sparks. I guess they'll try to claim (oh the harness was so brittle and while removing it, we discovered it is deteriorated (which is a false claim). In other words, they wanna scam Mbenz. I don't like to participate in shady practices. My car runs great, no issues, and I'd rather mechanics not touch it at all even if they are willing to give me a free part. It's under warranty with unlimited mileage until next year, so I've got nothing to worry about really. There is no need for me to ask them for anything. It's just them trying to find a warranty job that seems to be easy for them. He literally said word by word (we do at least one GLE and GLS every week like this).

@S_W222 I definitely agree in principle, and wouldn't participate in a shakedown either.

But I'll bet you do/will need a wiring harness.

My MB shop found them at 40k during a B service inspection. They just replaced them without asking.

Hello everyone,

I'm new to this forum and I really appreciate the information shared in this and other threads. It's a shame I didn't check here before changing the spark plugs.

I'm currently in a situation similar to many of you, and I mention @Owczar’s post because it describes symptoms very similar to what I'm experiencing.

I have a 2019 AMG GT43 4-door with 60000 km. I changed the spark plugs like I’ve done before (I already did this job around 30000 km). I disconnected the coils, removed them, replaced the spark plugs and reinstalled everything. When I start the car, everything seems normal, but after 2–3 seconds the check engine light comes on. At first I thought maybe I left a connector loose, so I checked everything quickly and started the car again. Same thing: after 2–3 seconds the check engine light appears.

I connected a diagnostic tool and it shows fault codes related to shorts. Looking more closely at the connectors, I noticed that the wires for coils 1, 2 and 3 were very cracked where they join the coil connector. I disconnected the battery, opened the harness, and all four wires for each coil had partially lost their insulation. As many of you mentioned, the insulation breaks down because of the heat.

While waiting for the new harness (10–15 days delivery), I rebuilt about 20–25 cm of wiring from each coil using new wires and new connectors After clearing the faults, the short circuit + low voltage errors come back even with just the ignition on. When starting the engine now, it runs fine for about 5–6 seconds, but then the check engine light comes back on again.

After reading this thread, I’m starting to suspect there might be a short inside the ECU. In @Owczar’s thread I can’t see the images of the ECU repair anymore. Could they be re-uploaded?Also, if the ECU is damaged, would it make sense that the engine initially runs fine for 2–3 seconds?

and after repairing the harness it now runs for about 5–6 seconds before the fault appears?

Thanks in advance.

Don't do it yourself, plugs 1-4 are easy, 5 is a bit of a pain, but number 6 requires a lot of disassembly around the turbo intake. I will not do it again, ever. I also read about the harness after I started, so I was very careful about how I handled it. I understand it is very pricey to replace. What I thought was going to be an afternoon's worth of work took about a week and I had StarDiag and WIS to assist. Not fun.

I'm to my second round of spark plugs soon (100k miles). Changed them at 50k. But I'm not changing them anytime soon. Just not going to temp fate on that wiring harness. Car runs fine, nice and smooth, and 50k on spark plugs, in my opinion, is ridiculous. They should last a lot longer than that in a modern engine!

Just tossing in some fuel carbon cleaner here and there for awhile. I know won't have any impact on carbon, but may help keep the plugs clean for longer.

It's my understanding that the 50 or 60,000 mile spark plug requirement is based on the tendency for the threads to seize.
There's a lot of thread between the combustion chamber and the seal, so it kind of makes sense. I guess.

Techron is a good additive, since it's essentially what Chevron uses to make their fuels top tier plus.

So did Mbenz fix this issue with a new harness part number by the way? Or are they just installing the same harness again waiting for the same car to come back for a 4000 USD job after another 50K miles!?

Do these issues affect the '24 GLS 580 too ?