Estimates last updated October 16, 2025

1991 Hyundai Sonata Thermostat Replacement Cost

The average cost for a 1991 Hyundai Sonata Thermostat Replacement is between $347 and $381.

Labor costs are estimated between $73 and $107 while parts are typically priced around $273.
This range does not include taxes and fees, and does not factor in your unique location. Related repairs may also be needed.

Thermostat Replacement

What is a thermostat?

The thermostat is a valve that helps control engine temperature. When the engine is cold, the thermostat stays closed to block coolant flow to the radiator, letting the engine warm up faster. Once the engine hits a certain temperature, the thermostat opens up, allowing coolant into the radiator, where it can cool off and keep the car from overheating. Thermostats can be controlled either mechanically or electronically.

What are the symptoms related to a bad thermostat?

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

Can I drive with a thermostat problem?

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.

How often do thermostats need to be replaced?

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.

Hyundai Thermostat Replacement Cost Estimates

The average cost for a Thermostat Replacement is between $347 and $381 but can vary from car to car.

Hyundai Sonata Thermostat Replacement Cost by Year

The average cost for a Hyundai Sonata Thermostat Replacement is between $347 and $381 but can vary from car to car.

Thermostat Replacement

Jump to:Commonly associated repairs Related OBD codes Cost estimates by vehicle Cost estimates by year

What is a thermostat?

What are the symptoms related to a bad thermostat?

Can I drive with a thermostat problem?

How often do thermostats need to be replaced?

How are thermostat issues diagnosed?

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.

How is a thermostat replaced?

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.

RepairPal recommendations for thermostat issues

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.

Can I replace the thermostat myself?

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.