Fuel Trim P0170 code

[dunky]

I keep getting this code in my 2003 C200 with M271 engine every time I cruise on the freeway.........never around town.

I have attended to as many vacuum lines and joining points as I can find. I have changed the petrol cap. I have cleaned the MAF. I have renewed the plugs and had the coils checked.....all OK.

I am about to change the fuel filter and if this does not fix it, I am looking at changing the upstream O2 sensor. But before I start to throw serious money at it, are there any other "try this first" options to look at?

The car has done 226K km.

Also is there a specific O2 sensor for the M271 engined C200 as compared to a C230? I have measured the tail and it is 900mm long.

Thanks Bob