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Photo DIY - SRS Seat Sensor replacement

363410 Views 157 Replies 78 Participants Last post by  caleno69
Your SRS light has come on and stays on, Also you noticed that the airbag off light stays on as well. Your car has never been in an accident, and your bags and belts have never deployed. You have some basic mechanical aptitude and don’t want to pay a dealer 500-800 $US for this repair. Read On…
It is not necessary but best to look for an independent mechanic shop that displays the 'pie', and have the codes read, this will tell you exactly what the problem is. Most will not charge for this service, and unless you have the MB software and can read the exact pin, you can’t do it..

Lets try and address the most common questions regarding this condition.

Q; - Is this a safety issue, the car still runs and a friend of mine who has a 2000 210 has a piece of electrical tape over the light and just drives it. Do the air bags still work?

A; No, Operation of all SRS devices are unpredictable, they may not fire at all, or they may fire for no reason

Q; How many Sensors are there and where are they?

A; Many.. The SRS system has a pre-tensioner in each seat belt, several air bags, sensors throughout the car to activate same, and a child sensor in the front passenger seat.

Q; Before I tear my car apart I would like to know as much as I can, what the module looks like and yhe functionality and design information

A; The child sensor adjusts air bag and seat belt level for the front passenger, and effects the functionality of several other devices.

Q; Is a sensor of some kind in the drivers seat?

A: No, a seat belt switch, and seat belt tensioner , and air bags.

Q; If I replace the Ass Mass will the light go off?

A; BE Clear Here, when you fix the fault, The Light WILL go off by itself if all faults have been corrected.

Q; How do I do this repair myself?

A; On the next posts, you will see a series of pics, but the procedure is,,,
1. At some point, have the codes read, by an inde, (PIC 1 ) as having the seat pad SRS light on will 'MASK' other ( emergency tensioning retractor belt and bag) faults.
2. Buy a NEW sensor first, (PIC 2 ) I have not seen ANY signs of wear ( wires, scraping, breaks ) at all. The sensor 'dots' fail with use/age. Sensors cost 140 US$ at MB.
3. TIP- Take the seat out of the car, period. If you have never done one, and are not an MB tech, or leather repair expert, 5 bolts and some clips and the seat is out. Don't even try to in car cover remove. you will me$$ it up!
4. TIP- Allow an extra 2 hours to shampoo the carpet under BOTH front seats. which is what you will do when you see all the gack under the seat. My car gets 'mini detailed' weekly and it still had lots of filth under the seat(s). Record amount of coins found so far.. $16.83 US$’
5. Unplug all wires from the connector block (PIC 6-7 )
6. Snap off the bolt covers (PIC 8 )
7. Remove all four 'Torx' type bolts (PIC 9-10 )that secure seat rails in place on the car floor.
8. Pull OUT not up on seat belt bolt cover(PIC 11 ) and remove bolt.
9. Release the wire cable (PIC 12 )
10. Tip the seat top down and remove the complete car seat from the car.
11. Remove the connector block and unplug the yellow sensor (PIC 13 ) connector from it's housing under the seat.
12.Take off the plastic side covers.
13. Loosen the 2 back bolts, (PIC 14-15 ) and remove the 2 front ones, This is optional and done to make the job easier.
14. Push down on the leather, (PIC 16-17 ) and pull out the bar that holds it to the frame.
15. Twist 90 degrees on the plastic ‘butterfly’ clips, (PIC 18-19-20 ) and release the leather cover.
16. Change out the sensor pad (PIC 21-22-23 ).
17. Once you have removed the sensor pad unit replacement is simply a reverse procedure. (PIC 24-25-26 )


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BTW There is not a sensor in the drivers seat since it is not needed. If your dash SRS light "STAYS ON" then it is the passenger seat

John May
Would another fault in the system not also light the warning light? For example dodgy wiring in a door airbag?
Hey all,

First off thanks for this great thread.

Before you go out and buy the sensor just try and check the condition of the lead 3 wire from the sensor. I removed the front part of the seat cover but left the back still on - while I tested. My brown wire had a fault and it was in a tricky spot. Past where the zip tie that lashes the wire to the seat frame - but just before the hole in the seat mesh / seat bottom. From the naked eye it looked fine - but it had broken - so test. I know I almost gave up but thought as long as I am in here.

I used a razor blade to carefully cut away the protective sheathing that covers all three wires and then used the blade again to carefully expose a small amount of wire to check for any loss in continuity. First I checked the white and it was fine then brown and it was the culprit.

Some tight spot soldering but in the end that did the trick and the light is off. There is no resetting or doing anything else - all good now. Ohh I did put protective tape on the metal hole where the line was cut and also put some sheathing on the brown and then the 3 wire.

It took me an hour and a half and I would not consider myself a mechanic - fairly handy with some tools but that's it.


Did you disconnect the battery out of interest? Guess with ignition off it might not have power anyway?

Also .... Have you seen the seat sensor bypass boxes you can buy? It keeps the airbag system activated and is plug and play.
Why would i not fit one of these?
through page 10. After reading the first 6 pages, I responded to a post of my early w210. I really looking for a by pass plug like your talking about for my 15 year old w210. One that keeps it activated.

Can you tell me where I can buy one, or whats involved to make one myself


Hi, sorry hadnt seen your question. There are a few on ebay but i am not sure they are a good idea. They make airbag system think someone always sat there which might not be a good idea if you had a child seat?
Final got around to fixing my SRS light. As others have mention in this post, the problem was a broken wire. The idiots at MB zip tied the wire to the seat pan so that there was no slack between the pan & sensor. So every time someone would sit in the seat it would stress the wire. I knew right away where the problem was. In fact the outer jacket was split right at the zip tie.

Anyway it was a major PITA to repair the wiring harness. Not for the faint of heart. I had to cut the wires right at the seat pan hole. I then had to solder the wires together with about an 1" of wire hanging out. I cut the jacket back far enough so I could slip some shrink tubing over each wire. It was a bitch. I about gave up on the repair. BTW checking the resistance aint easy either. For starters the resistance is very (left me scratching my head) high, I measured M ohms. I was expecting much lower resistance. Could have been because of smallish pins in the connector plug.

The E55 multicolored seats are a little tougher chore to disassemble. I did not remove the leather ( would have made life easy) cover. I couldn't freaking figure out how to get the damn cover off! I thought about totally disassembling the seat.

Thanks to all that contributed to this DIY it saved me lots O $$$!
Do you have any pics for any chance of where the damage was?
Oh geez, I ment to take pics of my repairs. My bad. The seat removal was straight forward and easy. Once out locate the conductor and you;ll see the hole where the wires go the through the seat pan. MB had zip tied the sensor multi-conductor wire to seat pan with no slack, no room to move with the cushion. Really poor workmanship. It was a no brainier. I'll bet there are plenty of other seats that have been done the same way. It's attention to detail...epic fail. How may $ have been spent replacing the sensor when I'll bet most failures are broken wires...
Just thought I would check, sounds like it will be obvious when I probe under there!
My car failed its mot yesterday on this sensor. Seems the 2013 mot rules now cover this in the UK.
Going to tackle it today.
I will look back when I get a second through this email but can anyone tell me if I need to reset with star after fixed or does it fix itself once repaired?
Many thanks!
Well this turned out not as hard as i thought.
Bit tricky taking it apart but once done I found the cable from the sensor was totally cut through! Very strange?
A quick solder and it was done. Maybe 4 hours taking it slow end to end.
Thanks for the tips and mine self reset itself thankfully.
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Mine had been cut right through the whole cable close to the underside of the seat padding. A place impossible to be affected by rubbing o the metal work. Why and by whom is the question. Strange indeed.
Mine was right through the cable - surgically clean - I would struggle to cut a cable as clean with wire cutters. I have no explanation of how it could come to be like that.
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