The computer in your 1998 Chevy truck generates and stores trouble codes based on the condition of the vehicle's emissions system. When there is a problem with the emissions system, the "service engine soon" light will come on in your truck. This will indicate the need for you to diagnose the exact nature of the problem and fix it. An auto parts store will diagnose the problem for free. A Chevy dealership will diagnose the problem, but for a small fee. The advantage of having a dealership do the diagnosis is that they can better pinpoint the problem using diagnostic software made specifically for your vehicle, whereas the auto parts store's diagnostic testing device will give you just a general idea of what is wrong (i.e., it will tell you several possible problems in the emissions system vs. the exact sensor that has failed). Once you've fixed the problem, you need to reset the computer.
Open the hood of your Chevy truck and loosen the retaining nut on the cable that runs to the negative battery terminal. The nut is on the cable's clamp, and must be loosened with a 10mm socket and socket wrench.
Slip the clamp off of the negative battery terminal and wait 15 minutes.
Reconnect the cable clamp and secure it to the terminal with a socket wrench.
Start the truck and wait for the engine light to shut off. This will indicate that your computer has been reset (and all trouble codes have been cleared).
