Ecoboost o2 sensor issue

[Jon250]

Morning all.

I’ve got a really intermittent issue with my daughters car focus 1.0 ecoboost 3 cyl. 65 plate. it flags up a P0036 HO2S heater control circuit bank 1 sensor 2, so xmas eve, I replaced the sensor, ran fine for a month, then came back, on further scanning I found her battery the original one, was only holding 50-60% charge, so replaced that, stop start now working, she forgot to tell me that part!  thinking that could be the cause, but 2 weeks later it’s back, it’s so intermittent it’s a pain. So going to look a bit deeper, does anyone have any suggestions on the issue. I wanted to check the voltage /resistance at the sensor plug, but struggling to find out which of the  four wires on the o2 sensor, are the heater circuit, earth, etc. colours are white, red, black and grey.  The fuse is ok, this is the only fault listed. Anyone able to help. Pic of old sensor enclosed 

thanks Jon