I have a similar problem as andrea. The low coolant light comes on only when am at speed (50 mph) for 10 mins or longer at that speed. Temp gauge says 160 degree? I was thinking first a bad sensor, and then maybe (Hot)air in the system? Thanks, Joe

How are you determining it's over heating ? Going by the temp gauge in the instrument cluster ? Did you hook up a scan tool to view coolant temp sensor parameters ? What the PCM - engine computer is seeing . Use a infrared thermometer to see what the engine temp is for real .


Do the cooling fans work ? If fans work an temp is in spec's , by scan tool an infrared thermometer testing the instrument cluster is faulty . GM had problems with stepper motors inside the instrument cluster model years 2003 to 2006 . The stepper motor is what drives the gauges .
https://www.youtube.com/watch?v=oTm2oDx-_GM

The PCM commands Low Speed Fans on under the following conditions: • Engine coolant temperature exceeds approximately 106°C (223°F).
• When A/C is requested and the ambient temperature is more than 50°C (122°F).
• A/C refrigerant pressure exceeds 1 310 kPa (190 psi).
• After the vehicle is shut off if the engine coolant temperature at key-off is more than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes.
The PCM commands High Speed Fans on under the following conditions:
• Engine coolant temperature reaches 110°C (230°F).
• A/C refrigerant pressure exceeds 1 655 kPa (240 psi).
• When certain DTCs set
When the request for fan activation is withdrawn, the fan may not turn OFF until the ignition switch is moved to the OFF position or the vehicle speed exceeds approximately 10 mph. This is to prevent a fan from cycling ON and OFF excessively at idle.

This is why you take your vehicle to a qualified repair shop . They have the knowledge an tools to diagnose correctly . Not just start replacing thing's not needed .

the lowering on the temp gauge indicates a lack of coolant around the bulb of the temp sender unit or a lack of flow around the cooling system
could be from low coolant level or blockage in the coolant system from forming ice ( 16 degrees is -8 degrees Celsius which is below freezing point of water)
I suggest that you check the antifreeze ratio in the cooling system and it should be no less than 50/50 to water
any thing less than 50 % antifreeze in the water will allow the coolant to freeze up which may be your problem
coolant condition should be checked every season as it does deteriorate over time and the % ratio will drop
topping up with normal water is another method of lowering the % point
check with a radiator shop or dealer service center to get the correct % of antifreeze for below freezing conditions and keep it at that point

Not real familiar with your engine, but generally you have two coolant temp sensors. One just runs the dash gauge. the other speaks to the computer and acts sort of like a choke. If the computer thinks the engine temp is hotter or cooler than it actually is, fuel metering will be wrong. The more likely culprit is the low fuel pressure. I have even seen a bad fuel cap cause something like this, as these systems operate under pressure I believe.

Have you checked your cooling fan to see if it is working propely? Let the car idle and get warm and listen for the fan to kick in (if they are electric) or just watch to see if the fan starts to spin faster if it is the old school type with a clutch. Have you replaced the thermostat? A bad one will let the temp rise since it opens too late and then the temp will drop off. But it won't due this only at a light and it will do it infrequently.

You have a air pocket in your cooling system!

The temp overheat could have been a false readiing due to a leak or low coolant level in the degas bottle(coolant expansion tank). The overheat signal is what induced the reduced power setting from the computer. Have your degas bottle checked at a dealership or by a competent mechcanic.

Check the sensor

It could be 3 things: Temp sensor, thermostat, or fan relay. Fans don't usually turn on until car has been idleing for a while. I first would change thermostat, then relay, then temp sensor. Coolant must be 50/50 mixture for water pump to have proper lubricant from antifreezer. Do not go any further than a 60water/40antifreeze mix

I would start with replacing your tranny fluid. black is no good. your tranny should hold about 7 qts of fluid. The tranny fluid runs through your radiator to help keep it cool. If your thermostat is stuck closed your eng coolant won't circulate and the coolant will get hotter inside your radiator. thus making your tranny fluid hotter and breaking it down. After time it will turn black from getting too hot. Let your car warm up to normal operating temperture then grab your upper radiator hose. It should be very warm. If not your thermostat is stuck closed.

check coolant level ,must be low ,chck for coolant loss