What a coolant sensor does and why it fails

Your engine coolant sensor (also called a coolant temperature sensor or CTS) sits in your engine block or radiator and tells your car's computer how hot the coolant is running. The computer uses that reading to adjust fuel mixture, ignition timing, and fan speed. When the sensor fails, your car can't read the temperature correctly, which means the engine runs too rich or too lean, the cooling fan may not kick on when it should, and you'll often see a check engine light.

Coolant sensors fail because they're exposed to extreme heat cycles and corrosive coolant. Over time the internal resistor breaks down or the electrical connector corrodes. A bad sensor usually shows up as a check engine code (most commonly P0115, P0116, P0117, or P0118 depending on which sensor is failing), rough idle, poor fuel economy, or the engine running hotter than normal on your dashboard gauge.

The good news: this is one of the more straightforward engine repairs you can do yourself if you're comfortable working under the hood. The sensor itself costs $15 to $50 depending on your vehicle, and the job typically takes 30 minutes to an hour once you've drained enough coolant to access it.

Key Takeaways

  • A coolant sensor tells your engine computer the temperature of the coolant so the computer can adjust fuel and cooling fan operation.
  • You'll need a socket wrench, a drain pan, new coolant, and a replacement sensor matched to your specific vehicle year and model.
  • The sensor sits either in the engine block or the radiator, and you must drain coolant below the sensor's location before removing it.
  • After installation, refill the coolant, clear the check engine code with a scanner, and run the engine to check for leaks.
  • If you're not comfortable draining coolant or working around the engine, a shop can do this job for $150 to $300 including parts and labor.

Locate your sensor and gather the right tools

Before you start, find out which coolant sensor your vehicle has and where it's located. Check your owner's manual or search your vehicle's year, make, and model plus "coolant sensor location" online. Most cars have the sensor screwed into the engine block near the thermostat housing, but some Fords, Hondas, and other brands put it in the radiator or upper radiator hose. Knowing the exact location saves you time and tells you how much coolant you'll need to drain.

Buy the correct replacement sensor for your vehicle — do not guess. Sensors are not universal; a sensor for a 2015 Toyota Camry will not work on a 2015 Honda Accord. Take your vehicle information to an auto parts store and ask them to look up the part number, or order online using your VIN (vehicle identification number). You'll also need a socket wrench or wrench set that fits the sensor (usually 19mm, 21mm, or 22mm), a drain pan, fresh coolant (the type your manual specifies), and a coolant temperature sensor socket if your sensor sits deep in a tight space.

Have a scanner tool on hand if you own one, or plan to visit a shop after installation to clear the check engine code. Some auto parts stores will clear the code for free after you buy the sensor from them.

Drain coolant to the sensor's level

Let your engine cool completely — do not open the cooling system while the engine is hot, as pressurized coolant can spray out and cause burns. Once the engine is cold, locate the radiator drain plug at the bottom of the radiator (usually a plastic valve you turn counterclockwise) and open it into your drain pan. You only need to drain coolant until the level drops below the sensor's location; you don't need to drain the entire system.

If your sensor is in the engine block, you may need to drain less coolant than if it's in the radiator. Check your manual or online guides for your specific vehicle to see how much to drain. If you're unsure, drain until coolant stops flowing, then open the sensor location and see if coolant spills out. If it does, you've drained enough. Close the radiator drain plug once you're done.

Remove the old sensor and install the new one

Unplug the electrical connector from the sensor by pressing the release tab or clip and gently pulling it away. Use your socket wrench to unscrew the sensor by turning counterclockwise. Have your drain pan ready in case a small amount of coolant spills out. Once the sensor is free, remove it completely and inspect the threads in the hole — if they look damaged or corroded, you may need to use a thread repair kit or have a shop handle the installation.

Before installing the new sensor, explore a thin coat of dielectric grease to the electrical connector pins to prevent corrosion. Screw the new sensor in by hand first to avoid cross-threading, then tighten it with your wrench. Don't over-tighten; snug is enough. Plug the electrical connector back in until you hear or feel it click into place.

Refill coolant and check for leaks

Pour the correct type of coolant back into the radiator until it reaches the fill line marked on the radiator or overflow tank. Start the engine and let it run for a few minutes while watching the sensor area for leaks. If coolant drips from around the sensor, turn off the engine, let it cool, and tighten the sensor a bit more. Once the engine reaches normal operating temperature, the cooling fan should kick on; if it doesn't, the new sensor may not be working or the code may still be stored in the computer.

Turn off the engine and let it cool again. Check the coolant level one more time and top it off if needed. Overfilled coolant can overflow into the overflow tank as the engine warms, so don't fill it above the line.

Clear the check engine code

If you have a code reader or scanner, connect it to your vehicle's OBD-II port (located under the steering wheel on the driver's side) and clear the stored code. The check engine light should turn off within a few seconds. If you don't have a scanner, many auto parts stores will clear the code for free, or you can drive the vehicle for 50 to 100 miles and the light may clear on its own if the sensor is working correctly.

After clearing the code, drive the vehicle normally for a few days and monitor the check engine light. If the light comes back on, the new sensor may be defective, or there may be another problem with the cooling system or engine computer. A shop can run diagnostics to pinpoint the issue.

When to take it to a shop instead

If the sensor is stuck or the threads are stripped, if you don't have the right tools, or if you're not comfortable working around the engine and coolant system, a repair shop can do this job for $150 to $300 including parts and labor. Some shops charge a diagnostic fee ($50 to $100) to confirm the sensor is the problem before they replace it, so ask about that upfront. Independent shops are often cheaper than dealerships for this repair.

If you drain coolant and find it's brown or rusty instead of bright green or orange, your cooling system may need a flush as well. A shop can advise you on whether that's necessary for your vehicle.

Frequently Asked Questions

Can I drive with a bad coolant sensor?

You can drive short distances, but not safely for long. A bad sensor means your engine can't regulate temperature or fuel mixture correctly, so you may overheat, get poor fuel economy, or experience rough idle. The longer you drive, the more risk you run of engine damage.

How do I know if it's the coolant sensor and not something else?

A code reader will tell you. If you see code P0115, P0116, P0117, or P0118, the sensor is the likely culprit. If you see a different code, the problem is elsewhere. Many auto parts stores will read your code for free.

Do I need to replace the coolant after draining it?

You can reuse the coolant you drained if it's clean and not discolored. If it looks brown, rusty, or cloudy, replace it with fresh coolant of the type your manual specifies. Never mix coolant types.

What if the new sensor doesn't fix the check engine light?

The code may still be stored in the computer even though the sensor is working. Use a code reader to clear it. If the light comes back on after a few days of driving, the new sensor may be defective, or there may be a wiring or connector problem. Have a shop diagnose it.

Is there a difference between engine block sensors and radiator sensors?

They do the same job but are located in different places. Engine block sensors are more common and usually easier to access. Radiator sensors are less common but the replacement process is the same. Always buy the sensor designed for your vehicle's location.