I've got a fault code p1131 lack of ho2s11 switches sensor indicates lean condition. Could someone explian this is it a bad o2 sensor and if so witch one?

What type of vehicle do you have? The sensor could be faulty, or you may have a lean condition which is why the sensor isn't picking up a change.

EDIT: Sorry, I missed that you put what type of vehicle you have in the subject. But the advice remains the same. Your best bet would be to take it to a garage that has the capability to monitor what the O2 sensors are reading at and if the upstream sensor is lazy or not reading anything at all.