1999 Hyundai Sonata Thermostat Replacement Cost

The bad thermostat can cause two different sets of problems, depending on whether it's stuck open or stuck closed. A thermostat that is stuck open can cause the engine to run colder than normal and turn on the check engine light. It may also cause poor fuel mileage and the heater to blow cool air. A thermostat that is stuck closed will cause the vehicle to overheat. If a new thermostat doesn't resolve engine temperature problems, the cooling system needs to be checked for other issues. » LEARN MORE: Signs of a bad thermostat, and how to fix it

If the thermostat is stuck closed, no, you should stop driving immediately; if you don't, the engine can quickly overheat. An overheating engine should be shut off, and the vehicle should be towed. Overheating can severely damage the engine after only a few minutes. If the thermostat is stuck open it may not cause engine failure, but you'll likely notice poor performance, an ineffective heater and bad fuel economy. It should be replaced at your earliest convenience.

It all depends on the type of car you have and your driving conditions. The thermostat does not wear the same way a tire does, but will eventually fail and require replacement. Often, a new thermostat is recommended as preventative maintenance any time the cooling system is serviced, such as during radiator hose replacement or a timing belt service.

Because there are several problems that can cause overheating, a technician will do a full inspection of the entire cooling system. Where applicable, any trouble codes stored in the engine's computer will be pulled and relevant diagnostics will be done. The technician may check engine temperature sensor and the thermostat opening temperature. If the engine has overheated, the shop will look for signs of engine damage. Modern engines commonly have plastic parts, such as intake manifolds and thermostat housings, that can warp from overheating. These parts, along with the head gasket, should be checked for leaks.

To replace the thermostat, the technician will drain the coolant from the system and remove the thermostat housing from the engine. The sealing surface between the thermostat housing and the engine is cleaned, and the thermostat gasket is replaced. Once the new thermostat is installed, the cooling system is filled with fresh coolant and the system is bled of any air, then rechecked to ensure a proper repair.

When replacing the thermostat, inspect the thermostat housing for corrosion. A corroded housing may prevent connected hoses from sealing properly, and could fail soon after replacement. Be sure to note the orientation of the thermostat. An incorrectly installed thermostat may not open, leading to problems soon after replacement. Consider replacing the related hoses, too. Refrain from using any “leak stopper” products in your cooling system. While they may seem to save money in the short term, their ability to stop leaks will also eventually clog the cooling system. This could affect the radiator, heater core and engine coolant passages.

The vast majority of thermostats only require simple tools to replace. However, the procedures vary from car to car. Before ordering parts or attempting this repair yourself, look for guides on how to replace the thermostat in your specific vehicle. Significant disassembly of the front of the motor, or hose specific tools may be needed. Special procedures may also be required to fill and bleed the air out of the cooling system. If you're unsure you have the right tools or experience to fix the problem, consider reaching out to a RepairPal Certified Shop.