If anyone else has this issue, check your alternator output.
My car is used infrequently, so I trickle charge my car when it’s laid up.
I had recently upgraded the radiator including a new electric thermo fan.
The 4WD Light would come on after driving for a while, usually if it had got hot enough to activate the new thermo fan or if the A\C cut in. My alternator output was only 12.7v under load, so I have deduced that the battery was slowly discharging while driving and any sudden electrical load was enough of a voltage fluctuation to trigger the ATTESSA computer into thinking there was a fault. (Code 21)
Most people will find that they have added extra electrical load with upgrades/modifications over time which the original electrical system wasn’t designed for.
I have upgraded to a ARD 150A Alternator from RHD Japan.
https://www.rhdjapan.com/ard-low-resistance-high-output-alternator-z31-r31-r32-r33-r34-c34-c35-wc34.html
Problem solved.
These are almost a straight fit, only modification is a larger electrical Lug is supplied and needs to be fitted to your wiring loom for the output side of the alternator.
Cheers
