Upon arrival, system is showing a code E2.1 "Retries exceeded (Flame failed to establish)." Attempted to fire off furnace to test operation. Inducer motor comes on as it should. Ignitor glows as it should. Gas valve opens as it should but then only stays on for approximately three seconds. Inspected flame sensor and it is not terribly dirty. Removed one burner to inspect for rust. Burner is extremely rusty causing the flame sense feature to not work. When ignition happens, the furnace does not get any signal for flame established. Replaced oem Trane steel burner with oem Trane stainless steel burners to prevent rusting. Test ran operation of system with no other issues found or suspected.