P0118 Code: Engine Coolant Temperature Sensor Circuit High

What happens if you don't fix it

Symptoms you can actually notice

• Gauge reads cold or erratic
• Hard start when cold
• Cooling fan runs constantly

What the computer is actually complaining about

The coolant temp sensor is a thermistor in the coolant passage. Its resistance drops as coolant warms, so the signal voltage drops too. A high signal tells the ECU the engine is cold when it is not, so it holds a rich mixture and keeps the fan in warm-up mode. An open wire or an air pocket drives that voltage to the top of its range.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner with live data

  Any $25 Bluetooth reader plus a free app shows coolant temp in degrees. Compare it against the dash gauge before you touch anything.

• Digital multimeter Optional

  A $12 meter reads the 5V reference at the plug and the resistance across the sensor.

• Replacement coolant temp sensor Optional

  Runs $15 to $45. Some cars use a separate sender for the gauge, so match the part number to the engine sensor.

• Coolant and a funnel Optional

  A gallon of the correct coolant runs $15 to $30. You will lose a cup when the sensor comes out.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Plug the scanner into the port under the dash on the driver side, above your left knee. Key to ON with the engine cold overnight, then open live data and read coolant temp next to intake air temp.

   Look at

   Whether coolant temp and intake air temp both match the outdoor temperature before the engine has run.

   Fine if

   Both read within 5F of the morning air. The sensor is honest and the wiring is intact.

   Broken if

   Coolant temp shows minus 40F, or sits at a fixed number while intake air temp tracks the weather. That is an open circuit.

   What you'll see Two temperature numbers that both start near the morning air temperature and climb together once the engine runs for a minute.

2. 2 Open the hood and find the coolant reservoir, a translucent plastic bottle near the front of the engine bay on the passenger side. Check the level against the cold fill line, then look at the radiator cap on the top of the radiator or the surge tank.

   Look at

   The coolant level in the bottle, and whether the level drops overnight with no puddle under the car.

   Fine if

   Level sits between the low and full marks, and it holds there for a week of daily driving.

   Broken if

   The bottle is low or empty, or the level drops again after a top-off. Low coolant leaves the sensor sitting in air and it reads cold.

   What you'll see A clear bottle with orange or green coolant visible at the cold line. A dry bottle with crusty residue at the top means it has been low for a while.

3. 3 Follow the upper radiator hose from the top of the radiator back to the engine. The coolant temp sensor screws into the metal housing where that hose meets the block, a brass or plastic body about the size of a spark plug with a two-wire plug on the top or side.

   Look at

   Pull the plug and look into both halves for green crust, water, or a pin pushed back out of its shell.

   Fine if

   Clean pins, a soft rubber seal, and a plug that clicks home when you push it on.

   Broken if

   Green or white corrosion, or a pin that slides backward. That costs nothing to clean and sets this code on its own.

   What you'll see Two flat pins in a row with bright metal showing, and a seal that still feels soft. A soft click when the latch closes tells you it seated.

4. 4 Key to ON with the plug off. Probe the two pins inside the harness plug for the 5V reference and ground with the meter. Then ohm across the two pins on the sensor itself with the engine cold.

   Look at

   Reference voltage at the harness, and the resistance value across the sensor at room temperature.

   Fine if

   About 5V on one pin and ground on the other, and the sensor reads a few thousand ohms cold, dropping as it warms.

   Broken if

   0V on the reference pin, or an open reading across the sensor. A dead reference means the wire is broken between the ECU and the plug.

   What you'll see A steady voltage number on the meter and a resistance that falls when you warm the sensor tip between your fingers.

5. 5 Let the engine cool completely, then crack the radiator cap to release pressure. Unplug the sensor on the engine side and back it out with a deep socket. Expect a cup of coolant. Thread the new sensor in by hand, snug it, top off the coolant, and run the engine with the cap off until the fan cycles once.

   Look at

   Coolant temp on the scanner against the dash gauge while the engine warms from cold to full temperature.

   Fine if

   The scanner climbs smoothly past 180F, the dash gauge rises with it, and the fan kicks on near 210F and shuts off again.

   Broken if

   The scanner stays cold while the gauge climbs, or the fan runs nonstop from a cold start. Air is trapped at the sensor or the sender is the wrong part.

   What you'll see A temperature number climbing in step with the dash needle, then the fan roaring once and stopping. Steam or a fast climb past 220F means stop.

What the repair costs

Reference range compiled from 2 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: editorial reference range, not a live parts quote · Data as of 2026-11

How to confirm it's really fixed

✓ Clear the code and let the car sit overnight. On the next cold start, coolant temp should match the morning air within 5F. Drive 20 minutes and confirm the scanner passes 180F while the dash gauge rises with it. If P0118 stays off through two warm-up cycles, the repair held.

Common questions