Hi all, I took my car in to check the codes because I had the CEL on. The two codes that came up are P0128 and P0136. I already researched the P0128 having to due with replacing the thermostat. I tried to research the code P0136 but I'm having a hard time understanding which oxygen sensor needs replacing. I have read the terms "pre-cat", upstream, downstream. I have looked under the hood and I can SEE both O2 sensors. I understand that the O2 sensor closest to me while looking down would be the upstream sensor.....and the O2 sensor closest to the ground is the downstream sensor. Is this correct???? Also, I really need to know which O2 sensor corresponds to the code P0136. I have learned that it means BANK #1 SENSOR #2. So which one is it.

I appreciate any help

P0135 is upstream of the Cat, so my guess the P0136 should be the downstream one.

I replaced both (upstream recently) and am pretty sure. You can also unscrew the sensor and look at it. Mine was pretty cooked (black with carbon) when I took it out.

I want to buy both sensors. I have managed to find the upstream sensor part number 003-542-73-18 through mymercedesparts.com but I can't seem to find the downstream sensor part number. Anyone know?

These?

Hello,
I've replaced both oxygen sensors, but the next day the code shows up again. Help, any ideas?