05 sonata, temp gauge will go to the middle when idiling, but drops to nothing when driving & no heat?

providing your coolant is full cause low coolant will cause no heat
and your temp gauge is normal in the middle then most likely u have a bad blend door actuator usually on the passenger side

most likely the sender for the temp gauge or not enough water in the cooling system radiator

check coolant level,thermostat stuck open you have to replace,when you driving wind cool off.

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

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

-----------
Temperature gauge sits over half way mark? http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html
----------
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
-------------
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
----------
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
-----------
Dodge neon gauges not working? http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

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

----------
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

-----------
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

-----------
These details will help.
Thanks.

Surely sounds like it. drain & refill with 50/50 coolant-anti freeze

Your thermostat is stuck open, not allowing the engine to heat properly.
murf427

First thing comes to mind is coolant level.
Temp gauge only measures temp of water...it can't read air temp.
If there's an air bubble that gets to the temp gage...you will indicate a lower temp.
I'd check for leaks...a water pump just starting to weep can cause up / down readings.

After that I'd look at the thermostat.
An experiment to try:
Prior to getting on the motorway, slide a piece of cardboard in front of the radiator and see if temp stays up.
I had a thermostat break in half once in winter...I had to do this trick to get enough heat to run my defrost....but running around in local traffic, it seemed to work fine.

If your problem was just the opposite...I'd be sending you to the lower radiator hose.

Hope this helps a little.

replace the thermostat.

Check dash gauges. Are they stepper motor type. Check internet for repair service if so. Around $125

it may be your thermostat, the gauge on the dash is a dummy gauge, there is only three positions that it sees, dead cold, operating temp, and hot, so even though the needle looks fine it just means it s closer to operating temp then it is to cold,