Cat C12 sometimes not starting when I press START

Check all your connection including the grounds.

 

The click you are hearing is the solenoid working, check the small wire on the large positive stud, should be a somewhat thin wire with a large connector since it attaches to the positive stud, 90% that, 10% your starter is actually bad, cross a wire or screwdriver from the positive on the starter to the smaller positive on the solenoid while the truck is in run and see if she starts, basically just doing a bench test....except in the truck, wayyy easier troubleshooting

 

Depending on the truck
from your start button it should go to the solenoid on the fire wall and then from the solenoid on the fire wall to the solenoid on the starter sometimes in the wiring harness an older truck you need to run a new wire from your solenoid on your firewall to your solenoid on your starter is usually cures your problem

 

That original wire goes through the wiring harness and it’s hard to track it down and less you just put a whole new wiring harness ,but every time I’ve done the new wire it’s fixed it !

 

To really answer your question yes age and corrosion but I don’t know the age of your truck !