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Common Symptoms Of A Faulty Thermostat

By R&D
Published on September 5th, 2024

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The thermostat controls heat exchange flow for the cooling system, primarily between the engine block and the radiator. It plays a pivotal role in achieving and maintaining optimum operating temperature as quickly as possible.

The thermostat is located between the engine and the radiator, usually on the water inlet on the engine block. When the thermostat fails, the entire engine cooling system can be affected in the form of reduced engine performance, the inability to maintain operating temperature and a shortened engine life. This article highlights the common symptoms of a failed or faulty thermostat, providing insight into potential problems and their underlying causes.

What Causes Thermostat Failures?

Mechanical issues: The thermostat will normally fail in either the fully open or closed position. When a thermostat is stuck closed, it does not allow coolant to flow from the radiator to the engine block water inlet, resulting in an overheating condition. When a thermostat is stuck open, it does not prevent flow from the radiator to the engine block inlet, leading to overcooling and preventing the system from reaching operating temperature. Both these cases affect engine performance, reduce fuel efficiency, increase emissions, and can damage components within the cooling system.

Contamination: Contaminants in the coolant, such as dirt or debris that cannot be cleaned regularly, can affect the proper operation of the thermostat, leading to malfunction.

Wear and Tear: Over time, the thermostat's mechanical components can wear out, leading to reduced efficiency or complete failure. Repeated opening and closing cycles, exposure to high temperatures, and corrosion can contribute to wear and tear.

Electrical Failure (for Electronic Thermostats): Electrical failures can occur in modern vehicles equipped with electronic thermostats or those integrated into the engine management system. Issues such as sensor malfunctions, wiring faults, or Engine Control Module (ECM) failures can lead to abnormal thermostat behavior.

Common Symptoms of a Faulty Thermostat

Malfunction Indicator Light (MIL) or Check Engine Light (CEL) Illuminates

A faulty thermostat may trigger the MIL on the dashboard to illuminate. This can occur if the engine temperature sensor detects abnormal temperature readings due to thermostat malfunction.

Irregular Operating Temperatures

When a thermostat is stuck in the closed position, it restricts coolant flow to the radiator, resulting in engine overheating. Engine overheating can present in a sudden rise in temperature or a gradual rise in temperature. This overheating can affect components within the cooling system, gaskets between cylinder heads and the engine block, and in uneven heat distribution, causing warpage of mating surfaces.

Conversely, when a thermostat is stuck in the open position, coolant flows continuously through the radiator, even when the engine is cold. This can cause the engine to run cooler than normal and prevent the ECM from reaching Closed Loop operation. The ECM will normally respond by providing extra fuel in an attempt to heat the engine to normal operating temperature. This results in reduced fuel efficiency, increased emissions, and poor cabin heater performance.

Coolant Leak and Loss

Faulty thermostat housings or gaskets can cause coolant leaks around the thermostat housing or engine block, which can lead to coolant loss. This can be observed in a concentration of corrosion, a pool of coolant, or a sweet smell coming from the thermostat housing area.

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Abnormal Noises

A thermostat may not immediately report temperature changes through the instrument cluster. Noises, however, may be heard in the form of hissing, humming, or even gurgling. These can occur while the engine is running or not, at operating temperature or not.

Identify A Faulty Thermostat with the Innova Scan Tool

The Innova Scan Tool quickly retrieves codes and code definitions, DTC severity level, and emission readiness status, as well as any Freeze Frame data associated with retrieved codes. Freeze Frame data normally provides a “snapshot” of operating conditions when a DTC is set, such as engine speed, engine coolant temperature, Open or Closed Loop Status, as well as Short and Long Fuel trim, among several other Parameter Identification Data (PIDs). Additionally, you can monitor these PIDs by way of Live Data as a vehicle warms up, cools down, or idles.