CEL comes and goes

check your oil level and coolant level first. if both are normal, come back

 

What are the codes that were read? What type of maintenance/repairs have you done recently?

 

Does your heater work properly?

Seems like you have a coolant control valve issue. You should look in the radiator itself and look at the coolant level, not at the reservoir. There is an engine area cover right in front of the car, black in color with I think 6 clips, remove the cover and the radiator cap will be accessible.

While the car is COOL, open the radiator cap and see if the fluid is low. Many times the coolant control valve gets stuck because the coolant is low. The coolant is like a lubricant and the valve can get stuck with low coolant level.

Fill the radiator with proper Toyota pink coolant, run the engine with the heater blasting on high and get all the air out of the system. Keep filling the radiator until the level won't drop anymore (no more air in the system). The bleeding of the air will take a while so don't put the engine cover back on just yet. In the next few days, monitor the fluid level and fill as needed. Once the fluid level stabilizes and doesn't drop anymore, then you can put the cover back on the car. Hopefully that will resolve the issue of the coolant control valve without having to replace it.

Low coolant can also be caused by a leak in the water pump. If there is a leak in the water pump, you would have to address that too.

 

The clips are replaceable and quite cheap. Leave the cover off for now.

So it seems if the coolant is full, what you need is a new coolant control valve.

 

Yeah that's the right place. Fill it while the engine is cold and measure (or estimate) how much goes in. Let the engine run for a minute or two (not enough to get hot) and check again to see if you can get anymore in. Repeat the whole procedure after it's been driven for a few days to confirm that it's really full.

 

Do what Uart has mentioned, a gallon of coolant at the dealer isn't that expensive. You should make sure the coolant is full and it might correct your problem. Also you can get the clips at the dealer too, it shouldn't be more than a couple bucks.

 

Yes full to the brim, try to get a good estimate of how much you have to add to get it to that level. I normally from that point turn on the car and get the heater going on high. The level should drop some as the air comes out and you just keep adding until the level stays put and doesn't drop anymore.