How to Replace Your Engine Coolant Temperature Sensor

What the coolant temperature sensor does and why it fails

Your engine coolant temperature sensor (often called the coolant temp sensor or CTS) sits in your engine block or cylinder head and measures how hot your coolant is running. It sends that reading to your engine computer, which uses it to adjust fuel mixture, ignition timing, and fan speed. When it fails, your engine runs too rich or too lean, your check engine light comes on, and your cooling fan may not kick in when it should.

These sensors fail because they live in hot, corrosive coolant. The internal resistor degrades over time, usually between 80,000 and 150,000 miles, though some fail much earlier if your coolant is old or contaminated. A failed sensor will throw a code like P0117 (temperature too low) or P0118 (temperature too high) because it's sending a signal that doesn't match what the engine is actually doing.

Key Takeaways

The coolant temperature sensor screws into your engine block or head and takes 15 to 45 minutes to replace once you locate it, depending on engine layout and how much coolant you need to drain.
You will lose some coolant when you remove the old sensor, so have fresh coolant on hand and know your vehicle's coolant type before you start.
The sensor itself costs $15 to $80 depending on your vehicle, but labor at a shop runs $150 to $300 because technicians have to drain coolant and sometimes remove other components to reach it.
If the sensor is stuck or corroded in place, you may need a socket wrench and penetrating oil, or a professional with an impact driver to avoid breaking the fitting.
After installation, refill your coolant to the correct level, clear the check engine code with a scanner, and let the engine warm up to confirm the fan cycles normally.

Locating your coolant temperature sensor

The sensor location varies by engine. On most vehicles it's screwed into the engine block near the thermostat housing, or into the cylinder head. Some cars have two sensors—one for the engine computer and one for the dashboard gauge. Your owner's manual or a service manual for your specific year and model will show the exact location.

To find it without a manual, look for a connector with one or two wires coming out of the engine block or head, usually near where the upper radiator hose connects. It will be a small cylindrical fitting, roughly the size of a spark plug. If you can't locate it visually, a repair manual or online forum for your vehicle model will have photos and the exact bolt location.

Tools and materials you will need

Gather these items before you start: a socket set (usually 19mm, 21mm, or 22mm depending on your vehicle), a wrench or ratchet, a drain pan, fresh coolant matching your vehicle's specification (check your owner's manual—do not mix types), a new coolant temperature sensor, and a gasket or O-ring if your sensor uses one (many do). Have penetrating oil like PB Blaster or Liquid Wrench on hand in case the sensor is stuck.

You will also want a coolant funnel, a jack and jack stands if you need to access the sensor from underneath, and a diagnostic scanner if you plan to clear the check engine code yourself. If you do not own a scanner, most auto parts stores will read and clear codes for free. Wear safety glasses and work gloves—coolant is toxic and hot coolant can cause serious burns.

Draining coolant and removing the old sensor

Let your engine cool completely before you touch anything. Locate your radiator drain plug at the bottom of the radiator and open it, letting coolant drain into your pan. You do not need to drain the entire system—just enough to drop the coolant level below the sensor location, usually a gallon or two. Close the drain plug when you are done.

Disconnect the electrical connector from the sensor by pressing the tab or clip and pulling straight back. Use your socket and ratchet to unscrew the sensor. If it is stuck, spray penetrating oil around the base and wait 10 to 15 minutes, then try again. Do not force it—if it will not budge, stop and consider having a shop remove it with an impact driver to avoid cracking the fitting.

Once the sensor is out, coolant will drip from the hole. Let it drain into your pan. Inspect the old sensor's threads and the hole in the engine for corrosion or debris. If the hole is dirty, wipe it clean with a rag.

Installing the new sensor and refilling coolant

If your new sensor comes with a rubber gasket or O-ring, install it on the sensor body. If it does not, check whether the old sensor had one—if so, your new sensor should too, and you may need to order the gasket separately. Screw the new sensor in by hand first to avoid cross-threading, then tighten it with your socket wrench. Do not over-tighten—snug is enough. Most sensors torque to 15 to 20 foot-pounds.

Reconnect the electrical connector, making sure it clicks into place. Pour fresh coolant into the radiator until it reaches the fill line on the overflow tank or the base of the filler neck. Start the engine and let it idle for a few minutes, watching for leaks around the sensor. If coolant drips from the sensor, turn off the engine, let it cool, and tighten the sensor another quarter turn.

Once the engine reaches normal operating temperature, the cooling fan should cycle on and off. If it does not, or if the check engine light stays on, you may have an installation problem or a wiring issue. Turn off the engine and let it cool before investigating further.

Clearing the check engine code

After the sensor is installed and the engine has warmed up, the check engine light may stay on for a drive cycle or two while the computer relearns. If it does not clear within a few hundred miles, use a diagnostic scanner to read the code. If the code is still P0117 or P0118, the new sensor may be faulty, or there may be a wiring problem between the sensor and the engine computer.

Most auto parts stores will scan and clear the code for free. If you own a scanner, connect it to the OBD-II port under your steering column, navigate to the codes menu, and select "clear codes." The light will go out when ready, but the code will return if the underlying problem is not fixed. If the light comes back on after clearing, the sensor or its wiring needs professional diagnosis.

When to call a professional

If the sensor is seized in place and will not budge after soaking with penetrating oil, a shop can remove it without damaging the engine block. If you are uncomfortable working around hot coolant or electrical connectors, or if your vehicle requires removing other components (like intake manifolds or hoses) to reach the sensor, a technician can do the job in an hour or two.

If the new sensor does not solve the problem—the light comes back on or the fan still does not cycle—the issue may be a corroded connector, a broken wire, or a computer fault. These require a scanner and electrical testing that are best left to a professional. Expect to pay $150 to $300 in labor for sensor replacement at a shop, plus the cost of the sensor itself.

Frequently Asked Questions

Can I drive with a bad coolant temperature sensor?

You can drive short distances, but not safely for long. A failed sensor causes the engine to run too rich (wasting fuel and fouling spark plugs) or too lean (risking overheating and detonation). The cooling fan may not turn on when needed, which can cause the engine to overheat. Have it replaced as soon as you can.

What coolant should I use to refill?

Check your owner's manual for the exact type—most modern vehicles use either orange (Dex-Cool), green, or pink coolant, and mixing types can cause corrosion. If you do not have your manual, call your dealership with your vehicle's year, make, and model. Never use plain water, and never mix old coolant with new.

Do I need to bleed air from the cooling system after replacing the sensor?

Most vehicles do not require formal bleeding if you only drained a small amount of coolant. Fill the radiator, start the engine, and let it idle until the fan cycles on. If your vehicle has a bleeder screw on the thermostat housing or heater hose, open it briefly while the engine warms to release trapped air. Check your manual for the location and procedure.

How do I know if the new sensor is working?

Start the engine and let it warm up. The cooling fan should turn on once the engine reaches normal operating temperature (usually around 195 to 220 degrees). You should hear it spin and feel air coming from the radiator. If the fan does not turn on after 10 minutes of idling, the sensor may be faulty or the wiring may be loose.

What if the sensor keeps failing after I replace it?

Repeated failures usually mean contaminated coolant or a wiring short that is damaging the sensor. Have your coolant flushed and refilled with fresh fluid, and have a technician check the connector and wiring for corrosion or damage. If the wiring is burned or corroded, it needs repair before a new sensor will last.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.