Both. Car has 4. Which one?

 

All of them. Please

 

See attachments.

 

I cannot get to the front left sensor on driver side as the space is to too tight to reach hand in. Have you ever done this job before and if so did you take the exhaust down to do it. I was able to replace the rear and both front and rear on passenger side. Now I have 3 new ones and 1 old.

 

21mm wrench will loosen it enough to undo it by hand.

 

Its 22mm and loosening it wasn't a problem as it wasn't that tight but getting it out and putting a new one in this position seems impossible without dropping the exhaust. Otherwise a much smaller hand is required.

 

I was focusing on beneath the car and just noticed the front (upstream) oxygen sensor beneath drivers side can be seen from under the hood but is still a very tight area that can't accommodate hands. Not sure what tool they use at the dealership but it looks like with the right tool, it can be changed fairly easy. Not sure what the mystery tool is but it will also need to put the new sensor back in place without a hand and will require a rather long extension reach from above. If anyone has any ideas or if you've done this before, I'd appreciate your knowledge.

 

You need a 22mm crows foot socket to get the front sensor out

 

Unbolt the motor mount and jack up the engine with jackstand.

 

Not sure what that would do for gaining access to the left upstream oxygen sensor. I just saw it under the hood from above but it'll take a long reach tool that can grab it, twist it out and twist new one back in. Don't know what that tool would look like though.

 

You tilt the engine to one side so you can reach the sensor from the top.

I don't know what else to say, I've done it without removing anything, and I don't exactly have girly hands either.

 

I've never attempted to lift and engine before and wouldn't know how to do it without messing something up.

 

This is it. I took it to a European garage today and they said they wouldn't be able to do it. You can't even get a socket on it because its pointing directly at the silver heat shield above it. The electrical connector on it is between the engine and the firewall out of reach of hand as well as if they never planned on anyone changing it when the car was built. s.

 

Ah, that would explain it. M278, I was thinking of the M273 engine.

It's likely you'll need to drop the exhaust for this one, or remove the motor mount on the driver side and let engine sag down to create room.

 

First off, Good Luck



Word to the WISE, replacing oxygen sensors on the 2013 - W221 will bring factory showroom performance back to the car you didn't realize was missing and it is quite amazing. However, the upstream sensor on drivers side as shown above cannot be changed easily; the other 3 are not so bad but are achievable with patients and the right tools. You can see the upstream driver side oxygen sensor under the hood very well, you just can't reach it by hand. Its also pointing towards the heat shield above it which means you'll have a hard time getting a socket on it. To do so, you'll need to cut the wire off at top of it even when using an oxygen sensor socket that's fully open down one side. The electrical connector is also between the engine and the firewall just below the heater control valve which means you cannot get your hands on it to unplug it.

To do this job I'd recommend either putting the car in the air and lower the exhaust in which case you would easily be able to change all 4 of them, or you have a kid that can help you because very small hands and a small arm are necessary for the driver side upstream oxygen sensor. I learned a lesson on this 1 hard to reach sensor, the factory obviously put it in a place inaccessible to an adult hand and arm. A kid however can reach it from below if you have one, if not, you may want to consider renting a neighbors kid who wants to learn something about working on cars. Who knows, maybe you'll inspire a future Mercedes technician.

To unplug the upstream driver side sensor, I found a pair of 15 inch vice grips at Harbor Freight. I used these to grab one side of the plug and clamping on it from under the hood from above, while using a very long flathead screw driver to disengage the clip then using a pair of 15 inch long needle nose pliers to unplug the other side (long vice grips would have been a better choice). Would have been better to get two pairs of the long vice grips instead of needle nose pliers, so keep that in mind. Then I almost couldn't plug the new one back in from below due to reach, in which case a small arm and hand will come in handy. You'll need to leave the vice grip clamped on the receiving plug to leverage pushing the sensor plug back into it by hand because you'll be using only fingers. But before plugging it in you'll need to put the new sensor back in the pipe. To do this, I dangled it from above while using fingers from below to slowly screw it in, a kid would be best for screwing it in from below and someone up top the twist the wire as its turning. If I ever do them again myself, the exhaust must come down. If I don't have access to a lift, then I would recruit a kid to help or else take the car to the dealer. I'm convinced the dealer tech lowers the exhaust to do this.

I tried all kind of tools for this job but found or would have found the ones below to help best:

15 inch vice grips - get 2 pair
22 MM stubby open end wrench
22 MM open end crows foot wrench
22 MM oxygen sensor sockets - one should be deep well fully opened down the side for upstream driver side sensor
3/8 Ratchet and a couple size extensions
3/8 Ratchet universal joint
Gorilla tape to hold socket and universal joint to extension
15 inch wire cutters would have been nice to cut the wire at top of sensor before attempting to remove it, so socket slides right on it.
15 inch needle nose pliers
Hammer to tap wrench when trying to loosen old sensors
Very long flathead screwdriver

All 4 of my sensors were fairly easy to loosen, so they were not stuck like many people experience, only they are in very awkward positions to reach. Dropping exhaust would be way easier for changing the oxygen sensors on this car if you can get the car raised high enough but you would need a power or air ratchet to remove hard to reach exhaust bolts.

If you have the money, I'd pay the dealer to do this but first make some unique marks on your 4 sensors so you can check to see if they really changed them all. You'll find them before (upstream) of the CAT and one after (downstream) of the CAT on both sides. As much work as this is, do your self a favor and buy OEM as you'll not want to do it over.