You can't reliably check engine codes on a 2003 Chevy 2500HD without a scan tool (OBD-II scanner). The method of using the key to trigger a diagnostic code readout on the dashboard that worked on older vehicles doesn't work consistently on vehicles from this era, and might not work at all. While some people claim to have gotten it to work, it's unreliable and depends on the specific vehicle's computer and configuration.
Relying on this method will likely lead to frustration and inaccurate results. The best and most reliable method is to use an OBD-II scanner. These are inexpensive to buy, even at auto parts stores. They'll give you the specific trouble codes, which you can then look up online to understand what the problem is.
