Thermostat how to check thermostat if it is working or not

check your coolant level if the level is good then you need to check thermostat if it's not the thermostat then it could be the water pump or the heater core

get the the thermostat checked.

not at all times can thermostat contol your cooling system, check for water leaks, water pump, radiator fans(check for speed) if all your cooling systems are good , replace your cylinder head gasket,

check the vehicle has plenty of coolant in, and check the fan is working once warm. then check the thermostat is working correctly. only way you can correctly do this is remove the thermostat, place in a bowl/jug nof boiling water, and see if it opens up when in the water, if it dosent move, replace thermostat. also check fan fuse

Thermostat

Test thermostat in a jug and pour boiling water on it , it should compress meaning it works. Check that the waterpunp is operating pushing water into the engine .Check that radiator water flows to the watrpump. Is tension of fanbelt of waterpump sufficient that it doenst slip. Lastly check that the radiator fan is working if has a electric fan. if this doesn’t help check if radiator or engine water passages isn’t blocked.

Most likely the thermostat. The heater bypasses the thermostat. If the thermostat is blocked the only cooling is via the heater matrix. To check start the engine with the heater off and put your hand on the thermostat housing when the thermostat opens you should feel the housing get hot. If it doesn't the thermostat is faulty.

how is the radiator and the thermostat?  Do the cooling fans come on?  Are you totally filling the cooling system and bleeding it?

first check and if the thermostat is working. take it out put it in hot water. the thermostat should open up. also put a temp gauge in the hot water so you know at what temp it opens up. start there. dont forget theres more fuses under the hood for the ac unit

Check the coolant level, do this with a cold engine. Or if you have recently used it. let it cool down for about 25 minutes before removing the radiator cap. if it is low, refill with coolant. If it does not, check your thermostat. Or better yet replace your thermostat. Be sure to get the right one . the same degrees as your old one. Just ask the teller to give you one for your engine size and car Model.And year.See where the radiator cap is. Then drain some coolant to where it will not spill from the radiator cap. Remove the hose nearest the radiator cap and you will get to the thermostat housing. Remove the housing and note how the thermostat is placed. Take of the thermostat housing and the old thermostat. Clean the housing and around the hole where the thermostat goes of any deposits. Get some grease and cover the new gasket with grease on the housing to hold it in place. replace the New thermostat and make sure it is flush with the engine and the housing. If it is off , you could break the housing upon tightening the housing.replace the hose and refill with coolant slowly to eliminate any air spaces.. Also check the coolant reservoir and fill it to the indicator line. Restart the car and chek for any leaks. Then check the heater after the engine heats up. If that does not do the job it could be the temperature sensor or the heater core.The heater core will take more time to check and clean or replace.

check your temperature sending unit and be sure it is working properly and then check to see that your thermostat is working properly if you are getting heat inside of the vehicle from the heater then this would be a good indication that your thermostat is working properly. also check your coolant level is correct.
code P0128 is coolant thermostat ( coolant temp below thermostat regulating temperature )