With Scan Tool

The HVAC programmer contains sophisticated self diagnostics that, when accessed with a scan tool, are designed to identify the source of any system malfunction as specifically as possible, including whether or not the malfunction is current or the non-current (history) type. When a fault is present the VF display panel will flash.

DISPLAYING DIAGNOSTIC TROUBLE CODES (DTCs)

The only way to display diagnostic trouble codes (DTCs) is by using a scan tool. There are no provisions for "ON-BOARD" diagnostics.