Toyota: Tempture gage goes up and when you turn the heaters on it goes back down after a mintue or so, coolant level, toyota celica

Question
I have a Toyota celica 2001. When driving on the motorway the tempture gage goes up and once you turn the heaters on it goes back down to normal. It has had the thermostat and head gasket replaced so is there anything else it could be. The car has a full service history and everything. On the dash though there is another light that goes on which about the the carbon omissions but that has recently been done also. And the petrol meter sometimes goes down afew bars then back up. I am not sure if it is something to do with that itself?
Could you please help as i am sick of taking it to places and paying for all sorts when the problem has still not been resolved.
thanks

Answer
Greetings from the USA, Lauren.

What you have is a mechanic that did not finish his job properly.

After servicing anything on the cooling system, the cooling system needs to have any air present in the passages removed, or, "bled out". Take your Celica back to that mechanic, and tell him to bleed all of the air out of your cooling system. When he tells you that he has no clue as how to do it, tell him this:

REMOVE THE COOLANT OVERFLOW BOTTLE RETAINER BOLT
RAISE THE COOLANT OVERFLOW BOTTLE UP, AND POSITION IT ON THE CONVIENTLY PLACED COOLANT BOTTLE RETAINER NEAR THE TOP RADIATOR SUPPORT
START THE CAR, TURN ON THE HEATER TO HOT AND MEDIUM FAN SPEED, AND OBSERVE THE COOLANT OVERFLOW BOTTLE FOR BUBBLES
ADD COOLANT AS THE COOLANT LEVEL DROPS
WHEN THE HEATER STARTS BLOWING HOT AIR CONTINOUSLY, REFIT THE BOTTLE BACK TO WHERE IT BELONGS.

This should get you fixed up straight away.