Coolant level sensor

not the pump but air in the system or a broken wire to the coolant level sensor
that sensor at the top of the radiator tank has to be completely immersed in coolant or the circuit is broken and the light comes on
same effect id the wire to that sensor is broken in the insulation

Low coolant level, failed thermostat, blocked radiator core or failed pump are all possible causes. Check them in the order given in the list as they become more complex and expensive to fix. Coolant level is easy enough to check, if low just top up at the reservoir. Whilst doing this check that the relief valve in the cap is OK and that the filler cap sits tightly when screwed down. For the thermostat drain the coolant (should be a bled valve for this purpose towards bottom of radiator) into a bucket and remove the 'stat housing and put the 'stat in a pan of simmering water. The 'stat plate should open up about 5mm. Take the pan from the heat and add cold water and the 'stat should close after about a quarter volume of cold water has been added. If the 'stat fails to open or close then replace it. Whilst the system is drained of coolant remove the top and bottom rubber hoses from the radiator. With a hose flush water into the top radiator opening and check if water exits freely from the bottom of the radiator. If there is a sign of blockage put the hose in the bottom opening and put the water on full pressure to back flush the radiator. Keep doing this until the water is clear. If everything up until now has been fine the last thing to look at is the water pump, either the pump has seized or there is a fault with impeller. When refilling the coolant system after any repair make sure that the interior heater is on maximum. When warming the engine up keep the heat on maximum and this will help displace air trapped in the system. Whilst warming up squeeze the radiator hoses regularly again to keep bubbles moving towards the reservoir filler. Keep the reservoir topped up as air bubbles work their way out. Only when the engine is nearly at running temperature tighten the filler cap. After having driven the car, let it cool off and check the coolant level, again top up as necessary.

The code says that engine temperature was not reached to allow closed loop operation. Basically the coolant temperature sensor did not inform the ECU that the engine had reached normal running temperature. 4 things to check: 1) coolant level 2) coolant sensor cable and connection 3) coolant sensor 4) thermostat The first two are fairly easy. To check the coolant sensor (3) drain the coolant into a bucket (you can reuse it when you refill). Put a voltmeter across the connector pins on the sensor. Now immerse the end of the sensor in a hot cup of water and again measure the voltage output. You are looking for a change either up of down in response to temperature change. If you see a change the sensor is OK, if not change it for a new one. Whilst the coolant is drained take out the thermostat and drop it into a pan of simmering water. The 'stat should pop open. Take the pan off the heat and after about 5 minutes the 'stat should pop closed, if it does it is OK. If the 'stat remains open then it is faulty and this is the source of the problem in that coolant circulation is unimpeded and the engine is is being kept below operating temperature.

Make sure the reservoir coolant tank is at 'full'. Start the car from cold and observe. After a couple of minutes when the engine has begun to drop in revs ( the coolant temperature sensor should tell the ECU to begin to shorten the injection cycle as the engine warms) the top of the engine and the coolant passage to the thermostat housing should become warm to the touch. When hot the top rubber hose to the radiator should become warm and then hot as coolant begins to circulate. If the top hose fails to get hot at any time then the thermostat is at fault. If you have electrically driven radiator fans these should now come on. If they fail to come an at all you need to check the coolant temperature sensor, the electrical motor fuses and the motor relay. Lastly check the motor itself (just run a 12v supply to it directly). As the ar continues to idle, with the fans running, the bottom return hose from the radiator to engine should also become warm. If the bottom hose fails to become warm this points to circulation problems. Either the radiator had become clogged with rust sludge and needs an extensive flushing out with a high pressure hose or the pump is not functioning as it should. Quickly check that the interior heater works by asking for 'full heat with fan on full'. If the interior heater works this indicates that the main coolant pump is probably OK. To check the thermostat more thoroughly, remove it from the car and drop it in a pan of near boiling water; the 'stat should pop open. As the pan cools, at 88 degree centigrade or so, the 'stat should pop closed. Failure to observe this tells you the 'stat is a dead and needs to be renewed.

I have seen this before...as a matter of fact, it actually happened to one of my cars...the temp would get extremely hot then all of a sudden drop to the normal range...(like a stuck stat), I did replace the stat and the problem continued...I knew however that the stat was not opening at the proper time, because when it did, I could watch my temp hand drop to normal. The problem was air in the system...took a while to get it all "burped" out, now it works perfectly. Keep an eye on the coolant levels and the resevoir fluctuation. This was the defining moment for me, the resevoir level dropped suddenly, I refilled and the problem went away. Good Luck! I hope you find this to be very helpful!

bad low coolant switch or still low on coolant.

Engine Cooling The cooling system components are the:

• engine coolant temperature sensor (ECT sensor) (12A648)
• radiator (8005)
• degas bottle (8A080)
• radiator draincock (8115)
• water pump (8501)
• water temperature indicator sender unit (10884)
• water thermostat (8575)
• dual speed fan motor assembly

The water pump circulates the coolant.
The water thermostat:

• controls the engine coolant temperature.
• allows quicker engine warm-up.

The degas bottle:

• provides a location for fill.
• contains coolant expansion and system pressurization.
• provides air separation during operation.
• replenishes the engine coolant to the system.

The fan motor (8C607):

• operates only when the ignition switch is in the RUN position.
• will not operate with the switch in the OFF position.

The engine coolant flows:

• from the lower radiator hose (8286) to the water pump.
• from the water pump to the engine block and the cylinder heads.

A closed water thermostat returns the engine coolant to the water pump; an open water thermostat allows the engine coolant to flow to the radiator.
Engine coolant provides freeze protection, boil protection, cooling efficiency and corrosion protection to the engine and cooling components. In order to obtain these protections, the engine coolant must be maintained at the correct concentration and fluid level in the degas bottle.
When adding engine coolant, use a 50/50 mixture of clean, drinkable water and engine coolant.
To maintain the integrity of the coolant and the cooling system:

The engine block is probably so low on water that the water doesn't get to the temperature probe. The only thing that is effecting the termperature sensor is the air inside the block. You need to refill the system with water. A car cooling system is constantly heating and cooling. Your radiator cap has a pressure release valve. The valve can get weak and let water out the overflow when it's not suppose to and can be part of the problem As the engine warms the coolant expands and the excess goes into the makeup tank. As the engine cools it sucks it from the makeup tank back into the engine. This is all done through the hose near the radiator cap. The amount of coolant you've lost could be caused by a leak in the system or in the water pump. When the engine is cold you should remove the radiator cap and refill the system with water. Also fill the makeup tank to the maximum level indicator. After you have filled the system then start the car and watch the guage and see if you're overheating. You may have to replace the thermostat which controls the coolant temperature. If you still get no temperature or if the coolant level wasn't low then you may have to change the temperature sensor.

Check the radiator and make sure that it is full of water. It can fool you sometimes after you made the repairs, and there could have been an air bubble in the system somewhere, and while you thought there was plenty of coolant, there may not have been.

Always keep the radiator cap off the radiator while filling after a repair like this until you know the stat has opened. You will know it has opened when the water level drops, or you feel warm water entering the radiator. Then replace the cap and fill the reservoir.

If you have gotten the stat hot, then I suggest replacing again as they usually fail closed, particularly when over heated.

If the belt is turning the water pump, then it doesn't need to be replaced.

Reply back if you have further questions.

The thermostat is a good place to start if the only problem you are having is little heat.
Before you start removing the stat, check the following things:
-is the coolant level full?
-is there coolant leaking onto the ground?
-are the heater lines that run coolant into the cabin hot?
-is there excessive white smoke coming from the exhaust?

if the level is full and there is no leaks or white smoke, and the heater lines aren't very warm, then change the thermostat....but be careful when you fill the system back up with coolant. there is a bleeder valve on a black post on the water pump housing that needs to be opened to get the air out of the system. if your van has rear heat, which most do, the air will be harder to get out. a good trick that often works is to take the hose off of the bleeder tower and fill more coolant up there. also drive the van a about 5-10 miles and the check the coolant level. do this a few times until your heaters (front and rear) are throwing good heat.

be patient with the bleeding of these, it takes some time.

I would indeed fix the cam sensor issue, however it obviously isn't your overheating issue. I would be sure there is no air within the system. If you changed the head gasket it more than likely got hot before, and likely for the same reason that it is now. I would without a doubt change the thermostat now, it is good practice any time you do any kind of major over haul, especially for the price. Start the car up and allow it to get warm slowly pull the cap off and "burp" the system. leave the cap off, and rev the car up, with it revved up (and t-stat open) the level within the coolant overflow tank should go down. If not, your not circulating water. Changing the water pump is a bit of a project but not horrible. Let me know if changing the t-stat helps. Even if you did change it, do it again. Also this time drill a small hole in the flat steel ring probley 1/4" hole. This will help a small amount of coolant flow, helping getting air out of the system. Thank you for using fixya.com