P0117 is an engine trouble code that points to a problem with your engine coolant temperature (ECT) sensor circuit. When this code sets, your car’s computer thinks the engine is extremely cold or the sensor signal is shorted low. That can throw off fuel mixture, idle quality, and cooling fan operation. If you’ve scanned your car and found P0117, you’re in the right place. Below you’ll learn what the code means, common causes, symptoms, how to diagnose it at home, and what repairs usually cost at a shop.
What Does P0117 Mean?
P0117 stands for “Engine Coolant Temperature Sensor Circuit Low Input.” In plain language, your ECU is seeing a voltage from the coolant temperature sensor that is lower than expected. Most ECT sensors are negative temperature coefficient (NTC) thermistors: resistance drops as temperature rises. A short to ground or internal sensor failure can make the ECU think the engine is extremely hot or the signal is simply out of range.
When this happens, the ECU often goes into a failsafe strategy. It may assume a default coolant temperature, enrich the fuel mixture, alter ignition timing, and command the radiator fans on. That’s why you may notice poor drivability, bad fuel economy, or a constantly running cooling fan when P0117 is active.
Quick Reference
- Code: P0117 – Engine Coolant Temperature (ECT) Sensor Circuit Low Input
- Severity: Moderate to high – can lead to poor drivability and potential engine damage
- Common Causes: Bad ECT sensor, wiring short to ground, corroded connector, low coolant
- Typical Symptoms: Hard starting, rich running, rough idle, cooling fans always on, poor MPG
- Usual Fix: Replace ECT sensor or repair wiring/connector; clear codes and retest
Real-World Example / Field Notes
In the shop, I see P0117 most often on higher-mileage cars where the coolant sensor sits in a plastic housing or aluminum thermostat neck. A common scenario: the customer complains that the temperature gauge is dead, the cooling fan runs constantly, and fuel economy dropped. Scan shows P0117 stored. A quick check usually reveals a cracked sensor connector or green, corroded terminals from old coolant or a minor leak. Replacing the ECT sensor and pigtail, bleeding the cooling system, and clearing the code usually has the car back to normal in under an hour.
Symptoms of P0117
- Check engine light on: The MIL/SES light will be illuminated and P0117 stored in memory.
- Poor fuel economy: The ECU may run a richer mixture, burning more fuel than necessary.
- Hard cold starting: Incorrect coolant temperature data can cause long crank or rough start, especially in cold weather.
- Rough idle or stalling: Wrong temperature input can upset fuel trims and idle speed control.
- Cooling fans always on: Many ECUs default to turning the radiator fans on when they “lose” ECT data.
- Temperature gauge acting strange: Gauge may read cold, hot, or not move at all depending on vehicle design.
- Black exhaust smoke or fuel smell: A rich mixture from incorrect temperature reading can cause visible smoke and fuel odor.
- Reduced engine performance: The ECU may limit power to protect the engine when sensor data is unreliable.
Common Causes of P0117
Most Common Causes
- Failed ECT sensor: Internal short or failure in the coolant temperature sensor is the top cause. The sensor’s resistance drops too low, pulling voltage down.
- Short to ground in wiring: Damaged insulation, chafed harness, or pinched wires can ground the signal wire, forcing a low-voltage reading.
- Corroded or damaged connector: Moisture intrusion, coolant leaks, or oil contamination at the sensor plug can cause incorrect readings or intermittent shorts.
- Low or contaminated coolant: If the sensor is not fully submerged in coolant, or there’s heavy rust/sludge, the reading can be erratic and trigger P0117.
- Recent engine work: After head gasket, radiator, or thermostat jobs, the sensor may be left unplugged, damaged, or its wiring pinched during reassembly.
Less Common Causes
- Faulty ECU/PCM: Internal failure in the engine control unit is rare but possible, especially if multiple sensor circuits show odd voltages.
- Bad sensor ground reference: Shared sensor grounds that are loose or corroded can skew voltage readings.
- Incorrect aftermarket sensor: A low-quality or wrong-spec ECT sensor can have the wrong resistance curve, confusing the ECU.
- Harness connector issues at ECU: Bent pins, corrosion, or poor contact at the ECU connector can mimic a wiring or sensor fault.
Diagnosis: Step-by-Step Guide
Basic tools that help with P0117 include an OBD2 scan tool (preferably with live data and Mode $06), a digital multimeter, a thermometer or infrared temp gun, and basic hand tools. A wiring diagram for your specific vehicle is also very useful.
- Confirm the code: Connect your scan tool, read stored and pending codes, and note freeze-frame data (coolant temp, RPM, speed) when P0117 set.
- Check coolant level and leaks: With the engine cool, verify the coolant level in the radiator and reservoir. Look for leaks around the thermostat housing, radiator, and hoses.
- Visual inspection of sensor and wiring: Locate the ECT sensor (usually near the thermostat or in the cylinder head). Inspect the connector for broken locks, corrosion, coolant intrusion, or damaged wires.
- Check live data: With key on, engine off, look at the ECT reading on the scan tool. Compare it to ambient temperature. If it shows an unrealistically low or high value, suspect the sensor or wiring.
- Wiggle test: While watching ECT data, gently move the harness and connector. If the reading jumps around, you likely have a wiring or connector issue.
- Voltage and ground test: Unplug the sensor. With key on, use a multimeter to check for proper reference voltage (often 5V) and ground at the connector. Refer to your wiring diagram for correct pins.
- Sensor resistance test: With the sensor removed (or unplugged if accessible), measure its resistance and compare it to a temperature/resistance chart for your vehicle or a service manual. You can use a thermometer or IR gun to know actual coolant or sensor temperature.
- Check for short to ground: With the sensor unplugged and ignition off, measure resistance from the signal wire to ground. A near-zero reading indicates a short that must be traced and repaired.
- Inspect ECU connector if needed: If wiring and sensor test good, inspect the ECU connector for bent pins, corrosion, or water intrusion.
- Clear codes and road test: After repairs, clear P0117, monitor ECT data during warm-up, and confirm the code does not return.
Pro tip: Compare ECT to intake air temperature (IAT) on a fully cold engine before starting. They should be within a few degrees of each other. A big difference often points directly to a bad coolant sensor or wiring.
Possible Fixes & Repair Costs
Most P0117 repairs are straightforward once you pinpoint the cause. Common fixes include replacing the ECT sensor, repairing or replacing the sensor connector (pigtail), fixing chafed wiring, topping off and bleeding the cooling system, or, in rare cases, replacing the ECU.
Typical repair cost ranges (parts + labor) at a shop:
- ECT sensor replacement: $80–$250 (sensor cost and access vary by engine design).
- Connector/pigtail repair: $100–$250 depending on harness condition and labor time.
- Wiring harness repair: $150–$400 if tracing and repairing a short is time-consuming.
- Coolant flush and bleed: $100–$200 if coolant is contaminated or low.
- ECU replacement/programming: $500–$1,200+ (rare and usually only after thorough testing).
Costs depend on your vehicle make, how difficult the sensor is to access, local labor rates, and whether related issues (like leaks or corroded connectors) are found at the same time.
Can I Still Drive With P0117?
You can often drive short distances with P0117, but it’s not ideal. The ECU may default to a rich mixture, which can foul spark plugs, damage the catalytic converter over time, and waste fuel. In some cars, the cooling fans will run constantly, which isn’t harmful but indicates the system is in failsafe mode. The bigger concern is that you may not get an accurate reading of actual engine temperature, making it easier to miss an overheating situation. It’s best to diagnose and repair P0117 as soon as possible.
What Happens If You Ignore P0117?
Ignoring P0117 can lead to long-term issues like carbon buildup, catalytic converter damage from running rich, and undetected overheating if the gauge or ECU can’t see true coolant temperature. You may also fail an emissions test due to an illuminated check engine light and incorrect fuel control.
Related Codes
- P0129 – Barometric Pressure Too Low
- P0127 – Intake Air Temperature Too High
- P0126 – Insufficient Coolant Temperature for Stable Operation
- P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0124 – Throttle/Pedal Position Sensor/Switch “A” Circuit Intermittent
- P0123 – Throttle/Pedal Position Sensor/Switch “A” Circuit High
- P0122 – Throttle/Pedal Position Sensor/Switch “A” Circuit Low
- P0121 – Throttle/Pedal Position Sensor/Switch “A” Circuit Range/Performance
- P0120 – Throttle/Pedal Position Sensor/Switch “A” Circuit
- P0119 – Engine Coolant Temperature Circuit Intermittent
Key Takeaways
- P0117 means the ECU sees a low-voltage signal from the coolant temperature sensor circuit.
- Most causes are simple: a bad ECT sensor, wiring short, or corroded connector.
- Symptoms include poor fuel economy, rough running, odd fan behavior, and a check engine light.
- Diagnosis requires a scan tool, basic electrical testing, and a visual inspection of the sensor and wiring.
- Repairs are usually affordable, and fixing P0117 helps protect your engine and catalytic converter.
Vehicles Commonly Affected by P0117
P0117 shows up across many brands because almost all modern vehicles use similar coolant temperature sensor designs. In the real world, this code is frequently seen on:
- GM vehicles: Chevrolet, GMC, Buick, and Cadillac trucks and cars, especially with aging plastic connectors.
- Ford and Lincoln: Focus, Fusion, F-150, Escape, and similar models with high mileage.
- Chrysler, Dodge, Jeep, Ram: Minivans, SUVs, and trucks where coolant leaks can corrode connectors.
- Toyota and Lexus: Corollas, Camrys, RAV4s, and Lexus sedans/SUVs with original sensors past 150k miles.
- Honda and Acura: Civics, Accords, CR-Vs, and related models, especially when aftermarket sensors are used.
- Hyundai and Kia: Compact cars and crossovers with aging sensors or harness issues.
It can appear on any OBD2-equipped vehicle, including European brands like BMW, Mercedes-Benz, and VW/Audi, but the diagnostic and repair approach is similar.
FAQ
Can I clear P0117 and keep driving if the car feels fine?
You can clear the code and the car may drive normally for a while, but if the underlying issue isn’t fixed, P0117 will usually return. Clearing without repairing only hides the problem temporarily and can lead to bigger issues later.
Is P0117 always caused by a bad coolant temperature sensor?
No. The sensor is the most common culprit, but wiring shorts, corroded connectors, low coolant, or even ECU problems can also trigger P0117. Testing the sensor and circuit before replacing parts is the best approach.
How do I know if the thermostat or the sensor is bad?
Watch ECT data on a scan tool during warm-up. If the temperature rises smoothly but the engine never reaches normal operating temp, suspect the thermostat. If the reading jumps, is way off from actual temperature, or doesn’t match IAT on a cold start, suspect the sensor or wiring.
Can low coolant cause P0117?
Yes, it can. If the sensor is not fully submerged in coolant, or there are air pockets in the system, the reading can be inaccurate and may trigger P0117 or related codes. Always verify coolant level and bleed the system if needed.
How urgent is it to fix P0117?
You usually don’t need a tow unless the engine is overheating or running very poorly, but you should address P0117 soon. Incorrect coolant temperature data affects fuel control, emissions, and engine protection. Fixing it promptly helps avoid more expensive repairs down the road.