hi all 2013 skoda yeti 2.0d
after scanning vahicle for a dpf fault found oxygen sensor(b1.s1) 0 volts,
when checkrd 12v pos at pin 4 but no neg at pin 3 earth from ecu wiring checked back ec all ok
can anyone tell me where the switched neg thru ecu comes from or is ecu gone

any help most appreciated