Not sure how old this posting is but the problem could be the lower intake gasket. I just had mine replaced. If its not under warranty the dealer said it would cost $800.
coolant level could be low not letting it circulate through the heater core, causing it to blow cold. Also, if the coolant level is low it will in turn overheat.
Temperature gauge may drop when heater turned on because additional coolant from heater core is introduced to cooling system - usually this coolant is cooler than what is currently circulating
The temperature of the coolant entering the heater core is controlled by the engine thermostat. The engine coolant thermostat is generally located at the engine end of the upper radiator hose.
When your car's heater is on, hot coolant from the engine is diverted to the heater core to provide warmth to the cabin. If the engine is overheating, it could indicate issues such as low coolant levels, a malfunctioning thermostat, or a failing water pump. These conditions can prevent the cooling system from effectively regulating engine temperature, causing the engine to overheat even when the heater is active. Additionally, if the heater core becomes clogged, it can restrict coolant flow and exacerbate overheating.
Low coolant? Thermostat not opening? Heater core restricted or plugged? Heater core airbound? Water pump not circulating coolant? Temperature control not working?
At the back of the engine on the coolant outlet that goes to the heater hoses on the firewall.
Low on coolant, stuck open thermostat, plugged heater core, faulty temperature blend door,
Need to check the coolant level in the Radiator, it is probley low on coolant. The heater core could be 1/2 stoped up. Can switch the hoses around on the heater core and go drive it, It will flush the heater core.
If you mean engine coolant is getting into the car, the heater core is leaking coolant causing the coolant level to drop and that causes the engine to over heat.
If the heater in your 2000 Volvo S40 is blowing cold air, it could be due to low coolant levels, a malfunctioning thermostat, or a clogged heater core. Low coolant can prevent the heater core from receiving enough hot fluid to generate warmth. A stuck thermostat may not allow the engine to reach its operating temperature, resulting in cold air. Finally, a clogged heater core can restrict airflow and heat transfer, causing the heater to blow cold.
Perhaps, but could also be: Low coolant? Bad thermostat? Heater core airbound, plugged? Heater temperature control cable frozen? Water pump not circulating coolant?
Check your coolant level. If its too low the sensor won't read the temperature of your coolant.