P013504 - oxygen sensor heater W176 petrol

[polnek]

Couple days ago check engine appeared for the first time, I read the codes using Autel AP200 and got:
P013504 -an electrical fault exists in the heater for oxygen sensor 1 (cylinder bank 1). An internal fault exists.

I've done lots of reading and in the meantime ordered another sensor (bosh). Based on the reading I've figured out it's 5 wire wideband with white and grey cables being for the heater. So, picked up multimeter and checked resistance, roughly 3 ohms. I wasn't sure if it's good or bad so when the new lambda sensor arrived i checked it and it also gives 3 ohms.
I've replaced the sensor with the new one and as you can guess it didn't solve the problem. I've tried to clear the codes but get error message that they can't be cleared.
Not sure what to do next, if there is an easy way to check what else could be wrong without having some sensor specific tool. I've checked fuses and all those under bonnet seem ok

 

[Uncle Benz]

There are two sensors on your car, one before the catalyst and one after. May be worth checking the other one. Some scan tools do get them muddled

 

[polnek]

There are two sensors on your car, one before the catalyst and one after. May be worth checking the other one. Some scan tools do get them muddled

Sounds like I may have been looking at the wrong sensor. There is another one in front of the car, but more accessible from the underneath. It's a different sensor because it's 4 wires not 5 and I couldn't get any resistance on it. Will get a new one and see if it helps.
I played a bit by removing one or both and check the codes, whichever I removed the codes were the same, even if both unplugged.

 

[polnek]

Actually Im not sure I was checking the right pins, does anybody know which ones are for heater in 4 wire lambda sensor in mercedes?

 

[Uncle Benz]

Sensor 1 is before the cat, sensor 2 is after the cat. Heater wires are usually both white, or 1 grey 1 white

 

[polnek]

I think for the 2nd sensor (4 wire) the heater is white and red. That's where i get resistance of around 3.5 ohms on both new and old.
I've got the new part and both old and new seems to give the same resistance. I've checked what voltage comes from the wires connecting with the sensor's heater wires and in both 1st and 2nd sensor are roughly the same, 11-12v.

I haven't replaced the 2nd sensor because all readings are the same and i don't have the tool yet to remove the 2nd one (less access) but I'm not sure what the issue is.

Am I misinterpreting the odb's results? The errors I get are "stored" but still can't get rid of them.