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There are a couple of different ways to connect and communicate with the truck using a device,

One way is to hook up a computer with the DIagnozer Software that Cat uses to recover codes and parameters

The other is using an automotive OBD2 Scanner, you can hook it up, it is usually located in the cup holder next to the regular computer connector.

See Caterpillar Forklift Fault Code Chart

When it is hooked up you can retrieve P codes even Transmission codes, it will point you in the general direction of where there may be an issue.

Very convenient for the do it your self type person that does not need to call out a service tech.
Don't forget to Clear Caterpillar Fofklift Codes.