Possible causes

– HO2S11 and HO2S21 sensors harness is open or shorted
– HO2S11 and HO2S21 sensors circuit connectors (wrong connection)
– Faulty Engine Control Module (ECM)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0040 BMW Description

The heated oxygen sensor (HO2S) monitor determines if the HO2S signal response for a fuel shift corresponds to the correct engine bank.