Temperature gauge fluctuates and the heater blows cold air

Bleed the air out of the system

The coolant sensor temperature is usually found just beneath the cylinder head, on the engine block.

You just disconnect the 1-2 wires and then undo it with a wrench.
The sensor will look like one of these:
1999 Chevy Lumina ltz 3 8 v6 coolant temperature sensor Google Search

Just be aware .. you may have TWO such sensors on your vehicle.

One sensor (usually with 3-4 electrical pins) is near the thermostat and works in conjunction with other sensors to regulate fuel/air according to engine temperature.

The other type of sensor is in the block near the cylinder head. It sends a signal to the temperature gauge on your dashboard ONLY. If your temp gauge constantly reads too hot or too cold .. this one is the culprit.
If it's a running fault .. speed .. rough running .. fuel. It may look like this:

Check out related help links with solutions to troubleshoot the problem:---
Click the link below:---
2004 Toyota Tundra: Oil Gauge and Temp Gauge Not Working?

http://howtobyme.blogspot.in/2012/05/2004-toyota-tundra-oil-gauge-and-temp.html

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Toyota 4Runner: Temperature Gauge Sensor http://technoanswers.blogspot.in/2012/04/toyota-4runner-temperature-gauge-sensor.html

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Temperature gauge sits over half way mark? http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html
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Heater blows intermittently and temp gauge goes erratic?

http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html
------------- No heat on Oldsmobile Silhouette? http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html
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Temperature gauge does not move and no heat when heater is ON? http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html
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Temperature gauge keeps fluctuating up and down close to red? http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html
----------- Temperature gauge on the dash quit working on Pontiac Grand Prix? http://technoanswers.blogspot.com/2012/01/temperature-gauge-on-dash-quit-working.html
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Dodge neon gauges not working? http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

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Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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1992 Lexus LS400: Engine overheats, Antifreeze out of reservoir and heater Blows Cold Air? http://whatisbyme.blogspot.in/2012/05/1992-lexus-ls400-engine-overheats.html

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These details will help.
Thanks.

For car running hot and overheating issues, I suggest to you go through this help links. Click this link below and go through the troubleshooting procedure to take care of the problems: ----

temp gauge goes erratic? http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html

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Temperature gauge keeps fluctuating up and down close to red? http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html

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Car runs hot? http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
Car freeze overnight and started overheating while driving?http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html-----------Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------overheating? http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------Car engine Overheats and transmission starts slipping?http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
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Temperature gauge does not move and no heat when heater is ON? http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

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This details will help.Thanks.

Temperature gauge should not be in the red unless you are talking about the temp control. Check hoses to heater core for a temp difference once the vehicle has reached operating temp. If a big differenced is noticed heater core may be plugged and may have to be flushed or replaced. Do the same with radiator hoses could also be a thermostat

Here's the thing. The ECU will shut down AC if the engine temperature is too hot. If the coolant temperature sensor is malfunctioning (lying to the ECU) telling it that the temperature is hotter than it actually is, the computer will turn the AC OFF.

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

The shut off valve on the heater core was left shut during the repair on your coolant system. the repairman forgot to re open the shut off valve . this is the problem. open the shut off valve and, the air will become hot again. as for the gauge fluctuations, the actual gauge may be malfunctioning.

check your coolant level on the radiator

maybe a blockage or thermostat is in backwards