P0118: Engine Coolant Temperature Sensor 1 Circuit High
Updated November 7, 2026
The coolant temperature sensor is sending a voltage so high that it would mean the engine is extremely cold, colder than is realistic. This is almost always an open circuit: an unplugged sensor, a broken wire or a failed sensor.
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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- A loose or unplugged sensor connector (often after cooling system work)
- A broken or corroded wire or connector, including the sensor ground
- A failed (open) coolant temperature sensor
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light
- Cooling fans running all the time on many vehicles
- Hard starting when warm, rough idle or poor fuel economy
- A temperature gauge that reads cold or doesn't move, on some vehicles
Is it safe to drive?
With this fault the engine computer and the dash gauge may not know the real temperature, so the gauge can't be trusted to warn you of overheating. Drive only short distances to get it fixed, and stop right away if you see steam, a temperature warning or loss of power.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- In live data, a coolant reading around -40 degrees with a warm engine points to an open circuit.
- With the key off, check that the sensor connector is fully seated and look for broken or corroded wires.
- Key on, engine off: briefly bridge the two connector terminals with a fused jumper. If the reading jumps to very hot, the wiring and computer are fine and the sensor is likely bad.
- Replace the sensor only with the engine cold; top up and bleed the cooling system afterward.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
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Questions
What does P0118 mean?
P0118 is the generic OBD2 code for "Engine Coolant Temperature Sensor 1 Circuit High." The coolant temperature sensor is sending a voltage so high that it would mean the engine is extremely cold, colder than is realistic. This is almost always an open circuit: an unplugged sensor, a broken wire or a failed sensor.
Is it safe to drive with P0118?
With this fault the engine computer and the dash gauge may not know the real temperature, so the gauge can't be trusted to warn you of overheating. Drive only short distances to get it fixed, and stop right away if you see steam, a temperature warning or loss of power.
How do I check P0118 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: In live data, a coolant reading around -40 degrees with a warm engine points to an open circuit. With the key off, check that the sensor connector is fully seated and look for broken or corroded wires. Key on, engine off: briefly bridge the two connector terminals with a fused jumper. If the reading jumps to very hot, the wiring and computer are fine and the sensor is likely bad. Replace the sensor only with the engine cold; top up and bleed the cooling system afterward.
Related codes
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