Assuming a properly made genuine resistor pack, that has been fitted correctly, really the only way it can fail is that it gets too hot.  It will normally have some sort of built-in thermal fuse which will go open circuit if too hot, as a safety measure.

 

This could be due to insufficient air flow to carry the normal amount of heat away e.g. due to a blockage or restriction of some sort, or the fan turning slower than it should.  You could check this by trying it on full speed (Demist), you should get a real gale.

 

Alternatively, it may be due to excessive power dissipation in the resistor due to it dropping too much voltage, e.g. a short circuit in the fan wiring which would place full battery volts across the resistor.  However, this would also stop the fan and you would notice this.

 

The normal dissipation of the resistor pack will be about 30 to 40 Watts.  The resistance would typically range from 0.5 ohms for 70% of full speed, to 3 ohms for 25% of full speed. As long as it has enough air flowing past, its surface temperature will remain acceptable and it will last for years.  Are you sure it is actually the resistor which is failing, and not for example its wiring harness connector ? The heat can tarnish contacts, which causes them to overheat and develop resistance of their own i.e. a vicious circle.  If the resistor pack really is failing, which part of it has gone open circuit ?

Thanks for your input again,

Wino.    Engine off 12.36v.   Engine running 14.72v  Anything significant there?

 

Austin.  I appreciate your time and obvious knowledge of electrics but its beyond me, owning an old Defender I’m a big spanner and hammer man.  For the Yeti it goes to the local main dealer for service/ repair, so I don’t know the condition of the parts or which actually failed.  I can only accept that it was the resistor and replaced twice.  The only question I can answer is yes there is a gale on full speed and when working a progressive increase in air volume on each step of fan speed up to max.

13 minutes ago, EP90 said:

Wino.    Engine off 12.36v.   Engine running 14.72v  Anything significant there?

That seems high, but it's high both off and running. Can you check the meter with another vehicle?

3 hours ago, EP90 said:

Engine off 12.36v.   Engine running 14.72v  Anything significant there?

 

3 hours ago, KenONeill said:

That seems high, but it's high both off and running. Can you check the meter with another vehicle?

 

That sounds normal to me, resting perhaps a little low.

Yeah, engine off figure is a bit low; 12.6V is what you'd hope to see on a fully charged battery. Engine running figure isn't high enough to suspect any fault with voltage regulation. 

OK guys thanks for your interest.  Back to the garage next week.  For the sake of future "Searches" I'll update post with, I hope, a positive outcome.

Just got the Yeti back, and as suggested by previous poster, dealer is now diagnosing a ‘worn fan’.  Replaced fan and another new resistor.  Fingers crossed....