2005 chevy equinox. The needle on the temp gauge is normally steady but from time to time swings back and forth from hot to cold and then normal, it also shows a code of p1245. What could be wrong??

Possible defective temperature sensor or wiring issue. Check to see if the heat coming out of the vents stays at a constant temperature when this problem is happening. If it goes from hot to warm/cold when the temp guage is messing up, you may have a head gasket isssue developiing

Check the coolant level. Sometimes when the coolant level is low, it will cause gauge fluctuations like you suggest, since air is getting into the cooling system. When air is in the system, it causes the thermostat to open and close frequently resulting is wild temp gauge swings.

What you are seeing is the thermostat opening and closing as the coolant heats up within the system. This, in itself, is normal. However, NORMALLY, you will not see such radically fluctuating temps on the gauge. This problem is usually due to a low coolant level in the system. This results in air being trapped in the system and that is what causes the radical gauge swings. Check the coolant level with the engine cold; fill the coolant and add coolant to the reservoir as needed and recheck.

If your temp gauge has only the cold (C) or hot (H) letters then the operating range would be the width of the needle under 1/2 way between c and h. If it remains steady at slightly over 1/2 way that would be just the setting of the gauge. How to get to know where the needle would sit at a specific position in relation to water temp. Do the following
Remove the temp sender unit and attach an earth wire to the body of the unit to earth. Place the sender unit in a container of water up to the threads .Put a mercury thermometer in the container beside the sender unit Apply heat to the container and watch the gauge needle move in relation to the thermometer reading. Operating temp will be about 80 degrees C on the thermometer and that would be about 1/2 way on the gauge.

Take to dealer and use your warranty.

Sounds Like A Problem With your thermostat. Good Luck

I would change the thermostat and if the gauge still acts up you may want to check the gauge, the fan is hooked up to a senor and will come on at a certain temp, usually when it gets hot. Hope this helped Tim

the fan comes on and off based on the "temperature sending unit" thats a thermal switch mounted on the radiator .. it is normal for the temperature to fluxuate as the fan turns on and off .. if it is allowed to get too hot before turning on then the temperature sending unit should be replaced ...while the car is moving the fan may never come on and isnt needed as long as air is flowing through the radiator .. as long as you are not loosing coolant then the temperatures are probably within proper range ... a sticking thermostat can also cause increased temperature range fluxuations .. that will show up as large swings of temp while driving .....

maybe the cam timing is off. If the engine was torn down and reassembled wrong, that would do it. Connect a vacuum gauge to the intake manifold and see what you get for vacuum readings.
It should be about 17 inches at idle. And, it should be steady.

1. NORMAL READING: Needle between 51-74 kPa (15-22 in-Hg) and holding steady.
   

1. NORMAL READING DURING RAPID ACCELERATION AND DECELERATION: When engine is rapidly accelerated (dotted needle), needle will drop to a low (not to zero) reading. When throttle is suddenly released, the needle will snap back up to a higher than normal figure.
   

1. NORMAL FOR HIGH-LIFT CAMSHAFT WITH LARGE OVERLAP: Needle will register as low as 51 kPa (15 in-Hg) but will be relatively steady. Some oscillation is normal.
   

1. WORN RINGS OR DILUTED OIL: When engine is accelerated (dotted needle), needle drops to 0 kPa (0 in-Hg). Upon deceleration, needle runs slightly above 74 kPa (22 in-Hg).
   

1. STICKING VALVES: When the needle (dotted) remains steady at a normal vacuum but occasionally flicks (sharp, fast movement) down and back about 13 kPa (4 in-Hg), one or more valves may be sticking.
   

1. BURNED OR WARPED VALVES: A regular, evenly-spaced, downscale flicking of the needle indicates one or more burned or warped valves. Insufficient hydraulic valve tappet clearance will also cause this reaction.
   

1. POOR VALVE SEATING: A small but regular downscale flicking can mean one or more valves are not seating.
   

1. WORN VALVE GUIDES: When the needle oscillates (swings back and forth) over about a 13 kPa (4 in-Hg) range at idle speed the valve guides could be worn. As engine speed increases, needle will become steady if guides are responsible.
   

1. WEAK VALVE SPRINGS: When the needle oscillation becomes more violent as engine rpm is increased, weak valve springs (6513) are indicated. The reading at idle could be relatively steady.
   

1. LATE VALVE TIMING: A steady but low reading could be caused by late valve timing.
   

1. IGNITION TIMING RETARDING: Retarded ignition timing will produce a steady but somewhat low reading.
   

1. INSUFFICIENT SPARK PLUG GAP: When spark plugs (12405) are gapped too close, a regular, small pulsation of the needle can occur.
   

1. INTAKE LEAK: A low, steady reading which can be caused by an intake manifold or throttle body gasket leak.
   

1. BLOWN HEAD GASKET: A regular drop of fair magnitude can be caused by a blown head gasket (6051) or warped cylinder head-to-cylinder block surface.
   

1. RESTRICTED EXHAUST SYSTEM: When the engine is first started and is idled, the reading may be normal but as the engine rpm is increased, the back pressure caused by a clogged muffler, kinked tailpipe or other concerns, will cause the needle to slowly drop to 0 kPa (0 in-Hg). The needle then may slowly rise. Excessive exhaust clogging will cause the needle to drop to a low point even if the engine is only idling.
   When vacuum leaks are indicated, search out and correct the cause. Excess air leaking into the system will upset the fuel mixture and cause concerns such as rough idle, missing on acceleration or burned valves. If the leak exists in an accessory unit such as the power brake booster (2005), the unit will not function correctly. ALWAYS FIX VACUUM LEAKS.
   

check the coolant temp sensor