My lad has a 2011 titanium 1.6 ti-vct, after having the timing belt done and a major service the eml light comes on. The lad who did it put the diagnostics on and it came back showing oxygen sensor, he reset the light but it came back again. He said that their are two sensors and best replace both. My question is as they are inline are they both upstream as cannot tell if one is behind cat and havent looked under car yet to see if sensors underneath also. if anyone knows the part numbers or can steer me in the right direction that would be great

One is pre-cat and one is post cat. The fault code should help narrow down which one is at fault.

Thanks for your reply Luke. Looking at the the engine do you know if the precat is nearest to cylinder one ? As they are both inline I cant tell which is which. Regards Jim

There are 2 different exhaust manifolds used on the Focus MK3  1.6 TI-VCT.
 

The 85 HP and 105 HP versions with manual transmission have a single catalytic converter with 2x O2 sensors (1x upstream and 1x downstream).

The 125 HP version with manual transmission and the 105 HP and 125 HP versions with automatic transmission have two catalytic converters with 4x O2 sensors (2x upstream and 2x downstream).

The DTC code should indicate which of the O2 sensors is causing the code.

Thankyou JW, its the 125hp version, so as I suspected they are both upstream, which is a big help. Im waiting for a scanner coming to see if I can get the codes up. The lad who looked said do both ( upstream ) as its difficult to tell which one faults although he did say the one nearest number one cylinder is usually the one. Im guessing that replacing both upstream should do the trick. Car has done 99k so safe to say its probably wise to change them. The downstream I will wait and see but before doing anything will try and get the codes again. Regards Jim