P0117: Engine Coolant Temperature Sensor 1 Circuit Low
Updated November 7, 2026
The coolant temperature sensor is sending a voltage so low that it would mean the engine is extremely hot, hotter than is realistic. This is usually an electrical fault: a shorted sensor or a signal wire touching ground.
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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 failed (internally shorted) coolant temperature sensor
- A signal wire shorted to ground, such as chafed insulation
- A damaged or corroded connector
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
- Hard starting or rough running when the engine is cold
- A temperature gauge reading hot, or erratic, on some vehicles
Is it safe to drive?
First make sure the engine isn't really overheating: if the gauge reads hot, you see steam or a warning light, stop and let it cool, and don't drive it until you know why. If the engine temperature is truly normal, it's usually OK to drive gently to a shop, but fix it soon.
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).
- Compare the coolant temperature in live data with how warm the engine really is; a reading at the very top of the scale points to a short.
- With the key off, unplug the sensor and inspect the connector and wiring for damage or rubbed-through insulation.
- Key on, engine off: with the sensor unplugged, the reading should drop to its coldest value. If it does, the wiring is likely fine and the sensor is suspect.
- 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 P0117 mean?
P0117 is the generic OBD2 code for "Engine Coolant Temperature Sensor 1 Circuit Low." The coolant temperature sensor is sending a voltage so low that it would mean the engine is extremely hot, hotter than is realistic. This is usually an electrical fault: a shorted sensor or a signal wire touching ground.
Is it safe to drive with P0117?
First make sure the engine isn't really overheating: if the gauge reads hot, you see steam or a warning light, stop and let it cool, and don't drive it until you know why. If the engine temperature is truly normal, it's usually OK to drive gently to a shop, but fix it soon.
How do I check P0117 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Compare the coolant temperature in live data with how warm the engine really is; a reading at the very top of the scale points to a short. With the key off, unplug the sensor and inspect the connector and wiring for damage or rubbed-through insulation. Key on, engine off: with the sensor unplugged, the reading should drop to its coldest value. If it does, the wiring is likely fine and the sensor is suspect. Replace the sensor only with the engine cold; top up and bleed the cooling system afterward.
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