After my adventure this weekend, I thought I'd post up the likely causes for engine overhating, or temperature gauge oddity.
1) gauge stays pegged low, no CEL
Low Coolant
Bad dash sensor
2) gauge stays low with a CEL of P0125
Low Coolant
Thermostat stuck open
3) gauge stays normal with a CEL of P0125 or P0115/7/8
Bad ECM sensor
4) gauge goes HoT while idle, but cools when driving
Low Coolant
Thermostat stuck closed
Water Pump
5) gauge goes HOT while driving, but cools when idling
Low Coolant
Fan Clutch
With all of these, you can plug in a scan tool and read the ECM's coolant sensor as it is a different sensor than the one feeding the dash gauge. Around 200* is a normal temperature. It is also advisable to run your heater at full blast until you can repair the issue as the heater draws in engine coolant to make heat.
1) gauge stays pegged low, no CEL
Low Coolant
Bad dash sensor
2) gauge stays low with a CEL of P0125
Low Coolant
Thermostat stuck open
3) gauge stays normal with a CEL of P0125 or P0115/7/8
Bad ECM sensor
4) gauge goes HoT while idle, but cools when driving
Low Coolant
Thermostat stuck closed
Water Pump
5) gauge goes HOT while driving, but cools when idling
Low Coolant
Fan Clutch
With all of these, you can plug in a scan tool and read the ECM's coolant sensor as it is a different sensor than the one feeding the dash gauge. Around 200* is a normal temperature. It is also advisable to run your heater at full blast until you can repair the issue as the heater draws in engine coolant to make heat.