Engine Thermostat
easy DIY~1h jobWritten in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026
What it does
The engine thermostat is a spring-loaded valve about the size of a hockey puck that sits where the coolant exits the engine toward the radiator. When the engine is cold, the thermostat stays closed — this lets the engine warm up quickly. Once the engine hits operating temperature (usually around 195–200°F), the thermostat opens like a flower, allowing coolant to flow to the radiator and cool down. If it sticks closed, the coolant can't escape — the engine overheats. If it sticks open permanently, the engine never reaches full operating temperature — it runs too cold, gets bad fuel economy, and the heater blows lukewarm air.
Fix-vs-Skip Money Panel
DIY Cost
~$25
parts only
Shop Cost
~$200
parts + labor
Estimates only — real prices vary by region, vehicle, and shop. Updated 2026.
Buy the part
Heads up: some links below are affiliate links (including Amazon Associates) — if you buy through them we may earn a small commission at no extra cost to you. Details: /disclosure.
So what do I do now?
Three honest paths for fixing engine thermostat — pick the one that fits your time, tools, and risk tolerance.
Door 1 — DIY it
Parts
- Thermostat (correct temperature rating) ($12–$30)
- Thermostat housing gasket or O-ring ($5–$15)
Tools
- Socket and ratchet set ($30–$60, one-time)
- Drain pan ($10–$20, one-time)
Consumables
Correct coolant type for your vehicle (never mix types), Distilled water for mixing coolant
Watch out for
- Only open the cooling system on a cold engine — hot coolant is pressurized and can cause serious burns
- Using the wrong coolant type, or mixing types, can cause sludge and damage the cooling system — match your owner's manual spec exactly
- Air pockets after refilling are common and can cause a false overheating scare — follow a proper bleed procedure (some cars have a dedicated bleed screw)
Skip DIY if: Skip DIY if you can't verify the correct coolant spec for your engine, or if the thermostat housing is buried under other components on your engine.
Heads up: some links below are affiliate links (including Amazon Associates) — if you buy through them we may earn a small commission at no extra cost to you. Details: /disclosure.
Door 2 — Buy the part
Already have someone lined up to install it (or handy friend), and just need the part?
Search Engine Thermostat on AmazonHeads up: some links below are affiliate links (including Amazon Associates) — if you buy through them we may earn a small commission at no extra cost to you. Details: /disclosure.
Door 3 — Go to a shop
Fair price band
$150–$250
parts + labor, typical US pricing
Ask the shop
- Are the parts OEM or aftermarket?
- Is the diagnostic fee waived if I approve the repair?
- Can I get an itemized estimate before you start?
- What warranty do you offer on the labor?
DIY tutorial
Click to load video — helps keep this page fast
Frequently asked questions
How do I know if my thermostat is stuck open or stuck closed?
Stuck closed: engine overheats rapidly after startup, temperature gauge goes to red. Stuck open: engine takes a very long time to warm up, temperature gauge stays below normal even after 10+ minutes of driving, heater blows cold/lukewarm air.
Can I drive without a thermostat?
Technically yes, but not recommended. Without a thermostat, coolant flows too freely and the engine never fully warms up. This causes increased fuel consumption, increased engine wear (engines run best at operating temperature), and no heat in the cabin. Always replace it.
Is thermostat replacement a beginner DIY job?
Usually yes — one of the easier cooling system jobs. The thermostat is typically accessible, held in by two bolts. The main risks: burning yourself if the engine is hot, and air bubbles in the cooling system afterward (burp the cooling system per your service manual).