Why Do They Fail?

Thermostats are subject to constant thermal cycling and can fail over time due to:

• Sticking open, often due to internal wear, spring fatigue, or debris

• Sticking closed, risking overheating and head gasket failure

• Housing cracks or seal failure, especially on plastic-bodied types

• Sensor failure, in electronically controlled units

• Coolant contamination, leading to corrosion or scale build-up

• Improper installation, such as airlocks or over-tightening housing bolts

Symptoms of a Failing Thermostat

• Engine overheating, especially on long drives or in traffic

• Poor cabin heating, caused by the thermostat stuck open

• Slow warm-up, leading to poor fuel economy and increased emissions

• Erratic temperature gauge, fluctuating or stuck in one position

• Check Engine Light (MIL), particularly on electronically controlled types

• Coolant leaks, often near the housing or pipe connections

Fitting Tips

1. Always replace the gasket or housing seal — never reuse the old one

2. Flush the cooling system before installation to remove contaminants

3. Ensure proper bleeding after refilling coolant — airlocks can cause false overheating

4. Use correct torque settings when fitting housings, especially plastic types

5. Check compatibility — some vehicles require temperature-controlled valves with integrated sensors or heaters