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Lambda oxygen sensor replacement issues

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Hi all,

Had the P0420 emmissions fault code come up which could have been anything from 1 to both sensors or the cat so started with the cheaper and easier option of replacing the sensors.  Bought a DEnso DOX-0109 as a direct replacement for the front pre-cat sensor, no apparent problems until I turned the ignition, within circa 20 seconds P0031 came up which is specific to the front sensor in regard to the heat sensor suggesting there's a power supply problem or I've managed to buy a faulty sensor.  tested the ohms across the black sensor cables on the sensor, came in pretty high at 16 ohms, old one was circa 3, new rear sensor also in at circa 6 ohms.  Looked at a few tutorials on Youtube and anything over 10 seems high, anyone know what it should be for a new Denso?  I've tried to comb the internet for some specs but can't find anything.  Also how do I test the supply?  I can see where the 2 black wires goe into plug from the car, turned ignition, put the positive probe on one of the pins, the other on negative terminal of Battery and there is a supply, tried the other pin and nothing, is that the fault or am I measuring the wrong thing?  From what i've seen 2 cables means one is earth and the other positive, should I just put the probes on both and if no supply then therein lies the fault?

Thanks in advance for any responses, I hate auto electrics!

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