When your vehicle’s diagnostic system detects code P0117, it means the engine control module (ECM) has received a coolant temperature reading that’s abnormally low. Your engine’s coolant temperature sensor monitors how hot your coolant is and reports this critical data to the ECM, which uses it to adjust fuel injection, ignition timing, and emissions controls. When this circuit reads too low, it throws a check engine light and can cause drivability problems.
What Does P0117 Mean?
P0117 is a generic OBD-II diagnostic trouble code that translates to “Engine Coolant Temperature Circuit Low.” The “circuit low” designation means the ECM is receiving a voltage signal from the coolant temperature sensor that’s below the expected range—typically below 0.1 volts on most vehicles.
Your engine coolant temperature sensor (also called the ECT sensor or coolant temp sensor) is a thermistor that changes electrical resistance based on coolant temperature. As coolant heats up, resistance decreases and voltage increases. When the ECM sees voltage that’s too low, it assumes the coolant is colder than it actually is, or that there’s an electrical problem in the sensor circuit.
This code is separate from P0118 (circuit high) and indicates a specific electrical or sensor malfunction rather than a general cooling system failure.
Common Symptoms
- Check Engine Light – The most obvious indicator; the light may be steady or intermittent
- Poor Cold Start Performance – Engine may crank slowly or hesitate when starting in cold weather
- Rough Idle – Engine may run rough or unstable at idle, especially when cold
- Reduced Fuel Economy – ECM may run in a “cold start” enrichment mode continuously, burning extra fuel
- Engine Overheating – If the sensor reads too low, the ECM may not activate the cooling fan when needed
- Black Smoke from Exhaust – Excessive fuel in the combustion chamber due to incorrect temperature readings
- Transmission Shifting Issues – Some vehicles use coolant temp data for transmission control
- No Symptoms at All – In some cases, the car drives normally but the code is stored
Possible Causes (Ranked by Frequency)
- Faulty Coolant Temperature Sensor – The most common cause; the sensor itself fails or becomes unreliable, sending incorrect voltage signals
- Corroded or Loose Sensor Connector – Oxidation, moisture, or poor connection at the ECT sensor plug prevents proper signal transmission
- Damaged Wiring or Connectors – Broken, pinched, or corroded wires in the ECT sensor circuit create open circuits or high resistance
- Low Coolant Level – Insufficient coolant can cause the sensor to read incorrectly if it’s not fully submerged
- Engine Control Module (ECM) Malfunction – Rarely, a faulty ECM fails to properly read the sensor signal
- Shorted Wiring to Ground – A short in the sensor circuit can pull the voltage too low
- Coolant Contamination – Dirty or degraded coolant may affect sensor accuracy
Diagnostic Steps
Step 1: Check Coolant Level and Condition
Start with the basics. Allow the engine to cool completely, then check the coolant level in the reservoir. If it’s low, top it off with the appropriate coolant type for your vehicle. Inspect the coolant color—it should be bright green, orange, pink, or blue depending on the type. Cloudy, rusty, or contaminated coolant can affect sensor readings.
Step 2: Visually Inspect the ECT Sensor and Wiring
Locate the coolant temperature sensor (usually mounted on the engine block, cylinder head, or intake manifold—consult your service manual). Look for:
- Corrosion or white/green oxidation on the connector pins
- Loose or damaged connector
- Cracked or pinched wires leading to the sensor
- Water or coolant leaking into the connector
Clean any corroded connectors with electrical contact cleaner and a small brush. Ensure the connector is fully seated.
Step 3: Test the Sensor with a Multimeter
With the engine off and cool, disconnect the ECT sensor connector. Set your multimeter to resistance (ohms) mode. Most coolant temperature sensors should read:
- At 68°F (20°C): approximately 5,000-10,000 ohms
- At 212°F (100°C): approximately 200-300 ohms
If your sensor reads open circuit (infinite resistance) or shows no change when heated, it’s faulty. Note: Exact values vary by sensor type—check your vehicle’s service manual for specifications.
Step 4: Check Wiring Continuity
With the sensor disconnected, use your multimeter to test continuity of the wires running from the sensor connector to the ECM. Set the meter to continuity mode and probe each wire. You should hear a beep indicating a complete circuit. If there’s no beep, the wire is broken or corroded.
Step 5: Test Voltage at the Sensor
Reconnect the sensor connector. Turn the ignition on (engine off). Using a voltmeter, probe the signal wire at the sensor connector. You should read approximately 0.5–4.5 volts depending on engine temperature. If the reading is stuck below 0.1 volts or won’t change, the sensor or circuit is faulty.
Step 6: Clear the Code and Test Drive
If you’ve cleaned connectors or topped off coolant, clear the code using an OBD-II scanner and take a test drive. If the code returns immediately, the sensor likely needs replacement.
Repair Cost Estimates
| Repair Type | Cost Range | Notes |
|---|---|---|
| ECT Sensor Replacement (DIY) | $30–$150 | Parts only; sensor costs vary by vehicle |
| ECT Sensor Replacement (Shop Labor) | $150–$400 | Labor + parts; some vehicles require intake manifold removal |
| Wiring Repair | $100–$300 | Splicing, re-routing, or connector replacement |
| Diagnostic Fee | $50–$150 | May be waived if you proceed with repair |
Budget-Friendly Tip: If you’re mechanically inclined, replacing the ECT sensor yourself can save $100–$250 in labor. Many sensors are accessible without major disassembly.
Can I Still Drive?
Severity: Moderate
Yes, you can typically drive with code P0117, but there are important caveats:
- Short Trips: The vehicle should be drivable for short distances, though you may experience rough idle, poor fuel economy, or hesitation.
- Overheating Risk: If the sensor reads too low, the ECM may not activate the cooling fan when the engine is actually hot. This can lead to engine overheating, which is serious. Monitor your temperature gauge closely.
- Emissions Failure: Your vehicle will likely fail an emissions test with this code active.
- Check Engine Light: The light will remain on, which is a visual reminder of the problem.
- Long-Term Damage: Prolonged driving with an inaccurate coolant temperature reading can cause engine damage if overheating occurs.
Recommendation: Diagnose and repair this code within a few days. It’s not an emergency, but it shouldn’t be ignored. If you notice the temperature gauge creeping into the red zone, pull over immediately and let the engine cool.
FAQ
Q: Is P0117 the same as P0118?
A: No. P0117 means the coolant temperature circuit is reading too low, while P0118 means it’s reading too high. Both indicate sensor or circuit problems, but the diagnostic approach differs. P0117 often points to an open circuit or failed sensor, while P0118 typically indicates a short to ground or stuck sensor.
Q: Can a low coolant level cause P0117?
A: Yes, it can. If the coolant level drops below the sensor, the sensor may read air temperature instead of coolant temperature, resulting in an abnormally low reading. Always check and top off your coolant level as a first troubleshooting step. However, if coolant level is normal and the code persists, the sensor itself is likely faulty.
Q: Will replacing the ECT sensor fix the code?
A: In most cases, yes—if the sensor is actually faulty. However, if the problem is corroded wiring or a bad connector, replacing the sensor won’t help. That’s why it’s important to inspect the connector and wiring first. If the connector is clean and tight but the code returns after sensor replacement, have the wiring tested by a professional.
Q: Can I drive to the mechanic with P0117?
A: Yes, you can drive short distances to a repair shop, but keep an eye on your temperature gauge. If the engine starts to overheat, pull over and let it cool. Avoid highway driving or heavy traffic where the engine works harder and generates more heat. It’s safer to have the vehicle towed if you’re concerned about overheating.
Q: How long does it take to replace a coolant temperature sensor?
A: Most ECT sensors take 30 minutes to 1.5 hours to replace, depending on sensor location and vehicle design. Some sensors are easily accessible on top of the engine, while others require removing the intake manifold or other components. Check your vehicle’s service manual for the specific location and procedure.