"Service Engine Soon" light came on, so I took it to AutoZone and they pulled the code PO128. They said it could either be a bad or sticking thermostat, or low coolant. Sure enough, the coolant in the reserve was nearly empty...so I bought some Prestone DexCool equivalent (I know I should swap out the Dex).
On my way home to put the coolant in, the light went out. I believe it takes 3 successful driving cycles without this code for the SES light to clear, so I'm hoping it was just a sticking thermostat and it unstuck. Everything seems to be fine now.
2001 Pontiac Bonneville SE - "SLE w/o the ugly" 3.05 gears, 180* thermostat, 35/20% tint, spoiler, leather, traction control, sunroof, leather shift/steering, automatic climate control. www.gillsystems.com