Hello,

I changed the oxygen sensor on my 1989 300E this weekend. While I was working on the car, I took pictures as progress was made and decided to make a write up on how to replace it. I am a visual learner and figure that there are at least a few others like me who would appreciate this visual write-up. I know a few other write-ups regarding oxygen sensor replacement have been made and posted to the forum and are great, but I was hoping to provide one with more pictures and detail.

Enjoy,
Rahul

P.S. I would appreciate it if the webmaster would add this write-up to the DIY Resources section of the website, if it is reasonable.

A Rahulio1989300E Guide: The Replacement of the O2 Sensor on an M103 Powered W124.

Why would I replace an oxygen sensor?
An oxygen sensor typically does not last the entire life of the car, but they can last quite a while before needing to be replaced. If you are experiencing rough engine running/idling, poor gas mileage or have an intermittent check engine light, the oxygen sensor could very well be the culprit. The oxygen sensor can be tested with a volt meter to determine if it is operating correctly.

Tools: I only needed two tools for this job, a small flat head screwdriver and a 7/8” open end wrench. The 7/8” is actually not the right size as it is slightly larger than the nut on the oxygen sensor. Since the old oxygen sensor was not seized in the exhaust pipe, I got by with this oversized wrench. The correct size is 22mm, but the (7/8” = 22.225mm) is the closet you can get with SAE wrenches)


The wrench…

Step 1: Raise the front of your car using jack stands or ramps. You may want to open your hood in order to let in more light to the underside of your car.


My 300E, with its guts showing…

Step 2: Before attempting to remove the sensor, go ahead and move your front passenger seat to the rear most position, remove the floor mat and carpet/padding combo piece to reveal the bare metal floor of your W124 and the oxygen sensor cables.


Cable running vertically on left side of image with connector placed in holding clip near the bottom.


Close-up of the rubber seal/grommet for the sensor wire which provides sealing from heat/noise/moisture.

Step 3. Now we can remove the sensor from the exhaust pipe by crawling underneath the car from the driver’s side. This was the most direct way to get to the sensor and gave me the angle I needed to get the wrench onto the sensor with room to turn it.


Old oxygen sensor before removal, connector can be seen traveling to inside of the car via previously mentioned seal/grommet.


Close-up of the old oxygen sensor in its port on the top of the exhaust pipe.


The old oxygen sensor has been removed and is ready to be disconnected and extracted.

Step 4. Now that the sensor is free from the exhaust pipe, it can be disconnected and removed from the car. First, push the rubber seal/grommet through to the outside of the car using your fingers or flathead screw driver. At first, I tried to extract the sensor via the inside of the car by pulling the sensor up, but the edges of the hex nut of the sensor were too large. I ended up disconnecting the sensor, pushing the seal/grommet though the hole from the inside and the retrieving the sensor by crawling under the car.


A sensor almost ready for removal…

Step 5. Compare and prepare the new sensor. The replacement oxygen sensor I used was manufactured by Denso and was fitted with the Mercedes-Benz proprietary connectors. I got this sensor for around $85 shipped from eBay. The Denso did not seem to have the same quality of wiring protection. I decided to modify it a little bit in order to prevent premature failure. Anti-seize should to be added to the threads of the new sensor in order to ease installation and future replacements.


Here, the old and carbon covered sensor is compared to the shiny new one. Make sure the nut is the same size on both sensors, ditto with the thread. Also, be sure your new sensor comes with a new washer.


Here, the length is found to be the same and the sensor comes with a replacement seal/grommet.


I did not like how the wire coatings were exposed so close to the sensor.


Two small and one large zip tie later, I prevented the cable shielding from sliding around and created a limit for the seal/grommet to slide up the cable. This limits the amount of wire that is exposed to the outside of the car. Make sure this seal/grommet is near the position of the old one on the old sensor since this determines how much cable is needed for the sensor to be connected to the exhaust pipe.


The Denso sensor came with a small pouch of anti-seize compound.


The anti-seize compound applied to the sensor…

Step 6. The new sensor actually fit thorough to the bottom of the car via the hole, this made things a bit simpler. You will want to push most of the new sensor’s wire through the hole to allow it to rotate freely while you are threading the new sensor into the exhaust pipe so that it does not bind up from all of the twisting motion. Once the new sensor is tight in the manifold, push the remaining wire back into the car from the bottom of the car. Place the new seal/grommet in the hole and reconnect the wires. Once everything is connected and installed, route the wire, replace the carpet and floor mat.


Sensor, seal/grommet, and most of the wire pushed through hole.


The new seal/grommet is a bit stiffer than the original one. If you notice, the seal/grommet has a groove in it, the metal fits in the groove which exposes half of the seal/grommet to the outside of the car and the other half to the inside. This is where the flat head screwdriver comes in handy; you can use it to force the groove to align with the metal.


This is an example of a correctly installed replacement seal/grommet.


Do not forget to reconnect the cables!


Routed wires with the connectors placed in the cradle.

Step 7. Test to make sure your car is operating well and enjoy! (You may want to test your duty cycle, MPG, etc.)

__________________
2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)