Thank you all for your interest and help. I intend to spend an hour or so doing a bit of homework based on your helpful tips before revisiting the vehicle.

If I understand PWM correctly it means, in simplictic terms, the frequency of switching remains the same no matter what is happening.

According to the NTK/NGK site, the battery voltage recorded on the sensor white wire was to the pump and I was only getting 2.95v on the green and yellow sensor wires which are to the heater. The difficulty I have is not knowing with clarity what I should be seeing. I will have a look further down stream on the wiring loom (with its different colours) and try and equate with the info I now have. I also do not understand why I have 6 loom wires going into a 5 pin socket. Also, why the "6th" wire is switching from 0.5v to 2v. Hopefully all will become clear on my next visit..

Interestingly, I have noted that the ECM will fault the heater and produce these DTC's if it does not see a closed loop situation within 30secs of start up based on the ECT sensor. So maybe the heater circuit is OK but the ECM is not seeing it due to a different fault.

Finally, should this heater be "on" all the time? I think not and remember my scanner telling me that the heater was "off" at some point in my investigations. On the next visit, I shall watch it carefully from start up.

Again, thanks for your interest but my gut feeling tells me I am out of my depth on this one but we will see..

Regards

Pete M