Just an update.
I had the CEM apart earlier. It was badly corroded as a result of what looks like electrolytic corrosion between pins up against the circuit board. This was only apparent after removing the CEM from its plastic casing.
The pictures show before and after.
I've cleaned all the pins and will pop it in the car tomorrow and see how it goes.
Looks like a new CEM might be needed.
https://ibb.co/1Lj0syt
https://ibb.co/yhqsWM8