CLS engine light on

[David557]

First post so hopefully in the correct section?
Picked up a 2011 CLS 350 petrol two months ago from a local independent dealer. Facelift model so not sure of the model number. Anyway superb car great condition with lots of extra s and with a mileage of 4500 that was the clincher. All services done by MB so car is 100%.
However have an engine light on and it has been back to a MB dealer for a diagnostic check and they have reported a faulty knock sensor and propose to change the two on the car. Cost will be into 4 figures and the company I bought it from will claim it back on their warranty. Car running perfectly I should add.
I am just a bit concerned by this fault and would appreciate some guidance from anyone who has knowledge of this and wether there are potential long term problems in store.
Thanks in anticipation

 

[KennyN]

Cost will be into 4 figures and the company I bought it from will claim it back on their warranty.

Just Googled the price of the knock sensor for the CLS and they are around £50 each.

Where the £1k + price has been conjured from i have no idea.

Time to get the car to a "reputable" indy so your location may be beneficial here so someone can recommend one that is close to you.

Kenny

 

[Tuercas viejas]

First post so hopefully in the correct section?
Picked up a 2011 CLS 350 petrol two months ago from a local independent dealer. Facelift model so not sure of the model number. Anyway superb car great condition with lots of extra s and with a mileage of 4500 that was the clincher. All services done by MB so car is 100%.
However have an engine light on and it has been back to a MB dealer for a diagnostic check and they have reported a faulty knock sensor and propose to change the two on the car. Cost will be into 4 figures and the company I bought it from will claim it back on their warranty. Car running perfectly I should add.
I am just a bit concerned by this fault and would appreciate some guidance from anyone who has knowledge of this and wether there are potential long term problems in store.
Thanks in anticipation

The knock sensors instantly retard the ignition and alter the fuel map a bit !, By itself it doesn't create too many issues but you will have at time as reduced power output as the car maybe is a little bit sluggish (possibly noticeable) in that hard uphill acceleration away from the lights! ( doing traffic light GP maybe!)
Long term is the fueling that might suffer in MPG.
Hope that helps
Tuercas viejas

 

[David557]

Thanks for the replies and I am just getting the hang of this forum. Thought I had replied earlier but can't see it, so if this is a duplicate sorry.

Thanks Kenny, I will update my profile to Ayrshire so not far from you. Don't use main dealers but as I am coming back to Mercedes after 5 years I would appreciate recommendations for specialists in the Glasgow and surrounding areas. Had thought about getting the next service done by a main dealer to keep the cars history, but this may change. Also appreciate you checking the cost of a sensor, I did like wise and can't get where the proposed cost is coming from. Nothing further will happen over the holiday weekend so hopefully I get a few comments on the post before Tuesday.

Tuercas, again thanks for your reply and when the light came on last week, I noticed the mileage drop on a regular run that I do. Now that it has been cleared mileage back to normal. Didn't notice performance change but to be honest I don't drive the car hard so if it did drop it wasn't that much.

Thanks
David

 

[Tuercas viejas]

David
Yes it will throw the consumption off because the code is a hard active code.

Now after several starts, usually cold soak starts, and the fault is intermittent or you don't drive it to a point where the knock sensors go active, then the fault goes "soft condition" and after about 5 incidence free starts & drive cycles the light will go eventually out by itself! The fault however will be recorded in stored faults for future reference in the OBD2 memory.

I am not a fan of doing a clear with a scanner or code reader simply because it opens all the OBD2 monitors and the car has to do a complete re-learn. This can take a lot of drive cycles to complete in some cases. It may in fact activate a dormant sensor which when subjected to a relearn activity faults out after several drive cycles.

In a professional repair shop scenario extra care has to be taken when a full clear is done . That extra care is to drive the car through its drive cycles and get all the monitors to close; then repeat a full cold start run test again!
This assures a quality over check scan in mode 6 & avoids that annoying scenario where car is handed back over to the owner/customer and in the morning the bloody light is back on! Grrr!
This I believe is the worst customer interrelationship issue!!! CEL back on after a handover.:fail
Cheers
Tuercas Viejas

 

[KennyN]

Mine was having trouble starting in the really cold weather a few months ago , so rather than changing things at random (glow plugs + relay) i took it to Merparts in Port Glasgow.

Merparts Ltd Independent Mercedes Benz Specialist

They ran a full diagnostic Star check , with a printout, on all the systems and the results found that the GP relay was knackered.

They charged me £80 for the test , which i thought was a wee bit pricey but only £120 to supply the genuine MB relay so there must have been a slight discount if they had performed the initial test. So all in £200 which i thought was fair.

Kenny

 

[David557]

Thanks for the replies. Have been advised today that one of the knock sensors is faulty and as I originally posted the recommendation is to replace both. Presume that's mainly down to the labour charges but a bit crazy that the perfectly good sensor will be scraped, thankfully I'm not paying as each one costs about £350, which I have been advised is the correct price.
Diasappointing start to my ownership of a top of the range model with only 6000 miles on it. Fingers crossed it's plain sailing from here.
Thanks again

 

[David557]

Have to correct my original post, when I was told of the fault originally what I heard was 'knock sensor' and when I saw it written down today it is 'Nox sensor' a bit of a difference but shows my lack of technical knowledge. Also explains the cost!
Anyway car in for repair today but with another fault. At the weekend the Airmatic Suspension dropped the back end completely, it did come back up when the engine started so I was able drive to the garage.
Thank goodness I have a warranty, but as I said in an earlier post it is very disappointing for a car with 6000 miles (albeit 6 years old) and a full MB service history. Once again fingers crossed for some trouble free motoring and some time to finally enjoy the car.
Thanks for the replies
David