these trucks had either an impco LP system or if it is tier2 or above it will probably have the spectrum system with the EPR regulator.
you can make it flash the error codes if you can access the diagnostic connector.
the older systems had either a flat or square connector plug marked terminals A B C D. if you jump the A and D terminals (engine off), then turn on the key and watch the MIL light (in some trucks that have the direction display on the OHG with the neutral light the MIL light is there). Once you have the diag plug jumpered you turn on the key and it will flash 3 times, pause, and then flash the code numbers with a 1 second pause between the digits, then a long pause and will start over repeating it.
If it has the spectrum system on it you will need the spectrum diagnostic module with dongle, no other way to diagnose it.
The diagnostic connector will either be under the floor plate on the RH side, or beside the distributor.
part number for the diagnostic ECU tester is 91868-02300.
cat also recommends having the service technical manual, an inductive tach and a digital vom.