My 2013 C300 is throwing P0133 and P0420.  The P0133 is the Bank 1 Sensor 1 oxygen sensor.  Can this be accessed from the top of the engine compartment or from underneath only?  Also, do you think replacing that sensor may resolve the P0420 Catalytic Converter issue?  Still under the emission warranty so hopefully if it doesn't that will cover it.  Thank you!


On the W204 C Class, you access the upstream oxygen sensor from teh top of the engine. There is little gap and you will need to use a long extension. I would spray WD40 or penetrating oil and let it sit for at least 10 min.

Yes there is a high chance P0420 will go away when you replace the O2 sensor. There is no way of knowing for certain. You are on the right path. Change the O2 sensor first and see if the code comes back on.

If the code does come back on, you may be looking at a new cataltyic converter. How many miles are on the car?

76K so I think it's still under the emission warranty.  Do  you know if that covers sensors too or just the converter?

Do you happen to have a diagram showing where Bank 1 Sensor 1 is located? Is it the same as the picture I saw on here for a different model?

If the dealer will fix the cat under warranty I would do that first. Then monitor if the other code comes back. They are not going to replace other parts free of charge but the O2 sensor is not that hard to replace and it is under $100. Depending on where you buy it.

It is similar to this set up.

mb o2 sensor

I have a feeling the dealer will tell me they have to fix the sensor first so they can get some $$ out of me lol  that's why I think we'll just do the sensor ourselves and then take it in! =)


Thanks for all your help, very appreciated!

You can also call a MB dealer and talk to a service adviser. See what they will say. The other thing to keep in mind is that they may charge you a diagnostic fee just to "confirm" that the problem is covered by warranty. If it is not covered by warranty you may be paying that fee. I agree with you. My recommandation would be to change sensor yourself and monitor the codes.

Something else you may want to consider is to do a back pressure exhaust test. There are gauges that plug into the O2 port and measure the pressure. If it is higher than recommended you have a clogged or partialy cloged catalytic converter. You may want to do the test espeically if the car is hesitating when going uphill.

Btw, can you take a few pictures while you change the sensor and post a DIY? It doesn't have to be long, but from what I can see there is not much help online this problem. To post a DIY login, go to My Account on the main menu, POSTs tab, them click Post a DIY.

I'm waiting for a call back from my local dealer and yeah, pretty sure they will charge me the $150 diag fee so I'm hoping if we can clear the sensor code they'll have to honor the warranty if the converter code stays on. lol The car runs fine btw. I had to have the front drive shaft replaced last fall, which is weird but it failed and luckily before it did more damage. The codes come on after this repair so I thought maybe they just didn't plug the sensor on the converter back in good so had them check it and they said it was connected properly, etc... I will def take photos and share them to the DIY!

If you clear the codes, all codes will be cleared. Also if there are no codes when they do the diagnostics, I really doubt that they admit to the converter being at fault. Which is unlikely that they will replace the cat under warranty. I would say leave the codes on. Just tell them to not worry about the other code.

If they diagnose and find the cat is bad and is covered under warranty, they may wave the daig. fee. Just ask.

Most likely just a bad oxygen sensor than the catalytic converter. Even though I have seen a few cases where the cat failed at around 60k. Looking forward to the pictures or a DIY. 

karkh, 2019-03-01

The correct location of sensor 1 bank 1?



Can you please let me know which on is sensor 1 bank on, so the pictures on the top are taken from behind the engine while the pictures on the bottom are taken from the left side of the engine. I appreciate if you can help me find the correct location?

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