WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC C0235 - C0237 Or P0609 Diagnostic Table (1 Of 2)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC C0235 - C0237 Or P0609 Diagnostic Table (2 Of 2)