I've never seen that code myself but the book says C018 = The battery on the electronic card has reached its lowest voltage.

It will be the A5 card under the armrest - the big silver card.

Be a good idea to make a note of all the parameters.

I have had these cards apart in the past but I haven't yet replaced a battery on one of them.

Obviously if you have the cover removed from the card take ESD precautions to avoid damaging the card.

There is usually a small round battery in a holder that is soldered on to the card - I know you can get them from specialist suppliers.