Oxygen sensor heater circuit problem

[MiniScot]

Can anyone help?
My 40K miles 02 mini runs fine but the engine emissions light comes on (started a couple of weeks ago) for no apparent reason. DTC scan codes 0030 and 0036 come up (generic HO2S bank 1 sen 1(2) heater circuit) after every engine start. I renewed the oxygen sensors (probably needlessly) before I got the scanner so should be OK and heater elements on the new sensors are approx 4.5 ohm. Not BMW sensors but are proper USA made replacements. I can’t find any info on the sensor heater circuit from either the Haynes manual or BMW TIS. With the sensor socket unplugged I have 4.5 volts to ground from one heater pin and 12.5 from the other. As far as I can tell all oxygen measurements from the scanner are fine and there no other engine problems such as rough running cold or hot. The sensor wires on old and new sensors are undamaged and intact.

This is driving me mad. Does anyone know what it means? Should I just ignore it and carry on with the emissions light on? Are there any other O2 sensor heater circuit bits I can hunt down (don’t seem to be any fuses or relays that I can see so guess they are controlled directly from the ECU). Is it an ECU problem? Does it need to be reprogrammed? Am I going in the wrong direction with this?

Any help or info much appreciated. Thanks.