GarageGuideEngine coolant temperature sensor
P0118 Code: Coolant Temperature Sensor Circuit High
P0118 stands for "Engine Coolant Temperature Sensor Circuit High." The coolant temperature sensor tells the computer how warm the engine is. The computer uses it to set the fuel mixture, idle speed, cooling fans and often the temperature gauge.
"Circuit high" means the sensor's signal voltage is too high, which the computer reads as an extremely cold engine, often around -40°F. That usually means an open circuit: an unplugged or damaged connector, broken wire, or failed sensor, not an actual cooling problem.
The engine is not necessarily overheating, but the computer is running blind on temperature. That hurts fuel mixture and starting, and your temperature gauge may not warn you if the engine really does overheat.
You can usually drive gently for short distances, but watch for steam, a sweet smell or reduced power, since the gauge may be wrong. Get it fixed soon.
Symptoms you might notice
- Check engine light on
- Cooling fans running all the time (many cars do this as a safety backup)
- Temperature gauge reading cold, jumping, or not moving
- Hard starting when the engine is warm
- Worse gas mileage, and sometimes black smoke from running rich
Common causes
Unplugged or loose connector
The sensor's plug may have come loose, especially after other work under the hood.
Damaged or corroded wiring
A broken, chafed or corroded wire between the sensor and the computer creates an open circuit.
Failed coolant temperature sensor
The sensor itself can fail internally. It is usually inexpensive.
Corroded sensor terminals
Green or white corrosion on the pins can block the signal.
Engine computer problem
Rare, but possible after the wiring and sensor check out.
Before you buy parts
P0118 is usually an electrical problem, not a real overheating problem. Check the connector and wiring before buying parts. Never remove a coolant sensor or open the cooling system while the engine is hot.
What to check
- With the engine off, find the coolant temperature sensor (usually near the thermostat housing) and check that its connector is fully plugged in.
- Look at the connector and wires for damage, melting, or corrosion on the pins.
- Check the coolant level when the engine is cold to rule out a real cooling problem.
- If you have a scan tool with live data, look at the coolant temperature reading. A reading around -40°F points to an open circuit.
- Unplugging and replugging the connector (engine off) sometimes clears corrosion; see if the code returns.
Possible repairs
Reconnect or repair the connector
Plugging in a loose connector or cleaning corroded pins is free or cheap.
Replace the coolant temperature sensor
Often an inexpensive part. Replace it only with the engine cold, and expect a little coolant to come out; top it off afterward.
Repair damaged wiring
Splicing or replacing a broken wire between the sensor and the computer.
Diagnose the computer
If the sensor and wiring are good, a shop can test the computer's sensor circuit.
Parts and tools that may help
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Frequently asked questions
What does P0118 mean?
The coolant temperature sensor circuit is reading too high a voltage, so the computer thinks the engine is extremely cold. It usually means a disconnected or damaged sensor or wire.
Can I drive with P0118?
Short, gentle trips are usually OK, but your temperature gauge may be wrong. Watch for steam or a sweet smell, and get it fixed soon.
Why are my cooling fans running all the time?
Many cars run the fans constantly when they lose the coolant temperature signal, as a safety measure. Fixing the sensor or wiring usually stops it.
What is the difference between P0117 and P0118?
P0117 means the signal is too low (computer thinks the engine is extremely hot, often a short circuit). P0118 means too high (thinks it is extremely cold, often an open circuit).