Bongo Fury

I have a Bongo Friendee 2L petrol 2004. The check-light came on and I had to pay 30 quid to get the OBD2 data read, so I'm interested in finding out if users can do this nowadays. But back to the sensor. This is a 4-wire type ( 5 ohm ) and I had to pay 300 quid for the exact mazda branded device. Frankly I now think this is nuts. Too late for me, but I wish I'd got an el cheapo device with a 5 ohm heater for 50 quid. The wires would be all wrong, but so what? A wire is just a wire.
Now, I replaced the sensor, but the light still wont go out. I checked the old sensor and the heater element was open-circuit which would trip the ECU, but now there isnt any voltage at the sensor plug/socket. So the sensor is not being powered, which would explain why the new sensor doesnt work either ( this is why you need a hand-held OBD2 reader of your own; a topic for later ). Now on the basis that lightning doesnt strike twice, I am puzzled as to why theres no power at the plug/socket any more since there must have been power there for the last 20 years! The wiring diagrams are not easy to get to grips with, so I dont know where the plug and socket gets its power from ie which fuse feeds it. Does anybody think he can help here?
Thanks

Not 100 percent sure I am right on this but I dont think the heater wires on the Bongo system would trigger a fault code.
Cheap code readers are available on Ebay that should allow you to scan for fueling faults that is if the 2004 Bongo is EOBD (I think it is) OBD is usually outside of EU.
With regard to the sensor voltage , this would be measured from the output of the sensor as the voltage is generated by the sensor so is not powered by the ECU.
Here is a link that may be usefull.

https://itstillruns.com/test-sensor-5193043.html.