If your Check Engine Light is on and your OBD-II scanner displays P0117, the Engine Control Module has detected an abnormally low-voltage signal from the Engine Coolant Temperature sensor circuit.
Despite the wording, P0117 does not always mean the engine is actually overheating. It means the ECM is receiving an electrical signal that suggests the coolant temperature is extremely high—sometimes beyond the realistic operating range of the engine.
The most common causes include a failed coolant temperature sensor, damaged wiring, a short to ground, connector contamination, or an actual overheating condition.
Because the Engine Coolant Temperature sensor affects fuel delivery, ignition timing, radiator fan operation, idle speed, and emissions control, P0117 should be diagnosed promptly.
Quick Answer
| Item | Information |
|---|---|
| Code | P0117 |
| Description | Engine Coolant Temperature Sensor Circuit Low Input |
| Severity | ⭐⭐⭐⭐☆ High |
| Safe to Drive? | Only after confirming the engine is not overheating |
| Most Common Causes | Failed ECT sensor, shorted wiring, overheating |
| Estimated Repair Cost | $20–$1,500+ |
What Does the P0117 Code Mean?
Diagnostic Trouble Code P0117 means the ECM has detected voltage from the Engine Coolant Temperature sensor circuit that is below the manufacturer’s acceptable range.
The ECT sensor is usually a thermistor whose resistance changes with temperature.
Generally:
- Cold coolant creates higher resistance and higher signal voltage.
- Hot coolant creates lower resistance and lower signal voltage.
When circuit voltage becomes extremely low, the ECM may interpret the signal as an excessively hot engine.
Common reasons include:
- Shorted ECT sensor
- Signal wire shorted to ground
- Failed sensor
- Corroded connector
- Coolant inside the connector
- Damaged wiring
- Actual engine overheating
What Does the Engine Coolant Temperature Sensor Do?
The Engine Coolant Temperature sensor monitors the temperature of coolant circulating through the engine.
The ECM uses this information to control:
- Fuel injector pulse width
- Cold-start enrichment
- Ignition timing
- Idle speed
- Radiator cooling fans
- Exhaust emissions
- Closed-loop fuel control
- Transmission shift strategy
- Engine protection modes
When the sensor indicates that the engine is hot, the ECM may reduce fuel enrichment, activate the cooling fans, alter ignition timing, or enter a protective operating mode.
What Does “Circuit Low Input” Mean?
The phrase circuit low input refers to electrical voltage—not necessarily low coolant temperature.
A low signal voltage tells the ECM that sensor resistance is very low.
The ECM may interpret this as:
- Extremely hot coolant
- Shorted sensor
- Signal wire shorted to ground
- Internal sensor failure
On some scan tools, coolant temperature may display an unrealistic value such as 280°F to 300°F or higher.
That does not automatically mean the engine has achieved lava status. It means the sensor circuit needs to be tested before the engine is accused of attempting atmospheric reentry.
Where Is the Coolant Temperature Sensor Located?
The ECT sensor is commonly installed near:
- Thermostat housing
- Cylinder head
- Intake manifold coolant passage
- Engine coolant outlet
- Upper radiator hose connection
Some vehicles use multiple temperature sensors.
One may send data to the ECM, while another controls the dashboard gauge. Other vehicles use a single dual-function sensor.
Confirm the correct sensor location with a service manual or wiring diagram before replacing parts.
Symptoms of a P0117 Code
Common symptoms include:
- Check Engine Light
- Temperature gauge reading hot
- Cooling fans running constantly
- Hard starting when cold
- Rough idle
- Engine hesitation
- Poor fuel economy
- Lean or rich air/fuel mixture
- Engine stalling
- Reduced engine power
- Limp mode
- Failed emissions inspection
If the engine is genuinely overheating, symptoms may also include:
- Steam from the engine compartment
- Coolant odor
- Coolant leaks
- Boiling coolant
- Loss of cabin heat
- Engine knocking
How Serious Is P0117?
Severity: ⭐⭐⭐⭐☆ High
P0117 can be caused by a simple electrical problem, but it can also appear during an actual overheating condition.
Potential consequences include:
- Engine overheating
- Blown head gasket
- Warped cylinder head
- Engine seizure
- Poor fuel control
- Hard starting
- Catalytic converter damage
- Incorrect cooling fan operation
The first priority is confirming whether the engine is actually overheating.
Can You Drive With P0117?
Do not continue driving until you verify the engine’s actual temperature.
You may be able to drive a short distance if:
- Coolant level is correct
- The engine is not overheating
- Cooling fans operate
- No coolant leaks are present
- Scan data confirms an electrical fault
Stop driving immediately if:
- Temperature gauge is in the red zone
- Steam appears
- Coolant is boiling
- Low coolant warning appears
- Engine loses power
- Knocking or pinging develops
- Coolant leaks heavily
Driving an overheating engine is an excellent way to turn a small sensor problem into a full engine replacement. Mechanics love this one weird trick. Your wallet does not.
Most Common Causes of P0117
| Cause | Likelihood |
| Failed coolant temperature sensor | ⭐⭐⭐⭐⭐ |
| Signal wire shorted to ground | ⭐⭐⭐⭐ |
| Corroded connector | ⭐⭐⭐⭐ |
| Coolant contamination in connector | ⭐⭐⭐ |
| Actual engine overheating | ⭐⭐⭐ |
| Damaged wiring harness | ⭐⭐⭐ |
| Low coolant level | ⭐⭐ |
| Failed thermostat | ⭐⭐ |
| Cooling fan failure | ⭐⭐ |
| Failed ECM | ⭐ |
Electrical faults are common, but actual cooling-system problems must always be ruled out first.
Most Common Repairs
| Repair | Typical Cost |
| Reconnect or clean sensor connector | Free–$75 |
| Replace coolant temperature sensor | $50–$250 |
| Repair wiring | $100–$400 |
| Replace damaged connector | $75–$300 |
| Cooling-system repair | $100–$1,500+ |
| Replace thermostat | $150–$500 |
| Replace cooling fan assembly | $300–$900 |
| ECM repair or replacement | $800–$2,000+ |
Repair cost depends heavily on whether the problem is electrical or caused by actual overheating.
Tools You May Need
Diagnosing P0117 may require:
- OBD-II scanner
- Scan tool with live data
- Infrared thermometer
- Digital multimeter
- Cooling-system pressure tester
- Wiring diagram
- Electrical contact cleaner
- Back-probe pins
- Basic hand tools
- Coolant catch pan
- Service manual
Step-by-Step P0117 Diagnosis
Step 1: Check for Actual Overheating
Before testing the sensor circuit, inspect the cooling system.
Check:
- Dashboard temperature gauge
- Coolant level
- Radiator fan operation
- Coolant leaks
- Steam
- Coolant odor
- Radiator hoses
- Overflow reservoir
Never remove the radiator cap while the engine is hot.
Hot pressurized coolant does not care that you watched a tutorial first.
Step 2: Compare Scan Data With Actual Temperature
Use a scan tool to observe Engine Coolant Temperature.
Compare the reading with:
- Ambient temperature when cold
- Intake Air Temperature
- Infrared thermometer reading
- Dashboard temperature gauge
After sitting overnight, coolant temperature and intake air temperature should be reasonably close.
If ambient temperature is 70°F but the scan tool reports 290°F before startup, the sensor circuit is clearly inaccurate.
Step 3: Read Freeze-Frame Data
Record:
- Coolant temperature
- Engine RPM
- Vehicle speed
- Intake air temperature
- Engine load
- Battery voltage
Freeze-frame data can show whether P0117 occurred:
- Immediately after startup
- During highway driving
- During overheating
- During an intermittent wiring fault
Step 4: Inspect the ECT Sensor Connector
Check the connector for:
- Green corrosion
- Bent terminals
- Loose pins
- Damaged locking tab
- Coolant intrusion
- Bare wiring
- Previous repair damage
Coolant can travel through a damaged sensor and contaminate the connector.
Step 5: Inspect the Wiring Harness
Follow the harness from the sensor toward the ECM.
Look for:
- Chafed insulation
- Melted wiring
- Pinched wires
- Rodent damage
- Contact with exhaust components
- Previous splice repairs
- Signal wire shorted to ground
A grounded signal circuit commonly causes P0117.
Step 6: Disconnect the Sensor and Watch Live Data
With the ignition on, disconnect the coolant temperature sensor.
On many vehicles, the scan-tool temperature should drop to an extremely cold reading, often around -40°F.
If disconnecting the sensor changes the reading from extremely hot to extremely cold, the sensor itself may be shorted internally.
If the reading remains extremely hot with the sensor disconnected, the signal wire may be shorted to ground.
Always follow the manufacturer’s diagnostic procedure.
Step 7: Test Sensor Resistance
Disconnect the ECT sensor and measure resistance using a multimeter.
Resistance should generally:
- Be higher when cold
- Decrease smoothly as temperature rises
Compare the results with the manufacturer’s temperature-versus-resistance chart.
A sensor showing near-zero resistance when cold may be internally shorted.
Step 8: Check the Signal and Ground Circuits
Verify:
- Reference voltage
- Sensor ground integrity
- Signal circuit continuity
- No short to ground
- No short to another circuit
- ECM connector condition
A damaged sensor ground usually causes high-voltage readings, while a signal wire shorted to ground commonly causes low-voltage readings.
Step 9: Diagnose the Cooling System
If the temperature reading is accurate and the engine is genuinely overheating, inspect:
- Coolant level
- Thermostat
- Radiator
- Water pump
- Cooling fans
- Fan relays
- Radiator cap
- Head gasket
- Coolant passages
P0117 may be reporting a real temperature problem rather than an electrical fault.
Step 10: Verify the Repair
After completing repairs:
- Refill and bleed the cooling system if opened.
- Reconnect all electrical components.
- Clear the code.
- Start the engine cold.
- Monitor coolant temperature.
- Confirm the reading rises gradually.
- Verify thermostat operation.
- Confirm cooling fan operation.
- Road test the vehicle.
- Rescan for stored or pending codes.
The temperature reading should rise smoothly without sudden jumps.
Scan-Tool Clues for P0117
Useful clues include:
| Scan Reading | Possible Cause |
| Extremely high temperature before startup | Shorted sensor or wiring |
| Reading changes when connector is unplugged | Failed sensor likely |
| Reading stays extremely high unplugged | Signal wire shorted to ground |
| Actual engine is overheating | Cooling-system fault |
| Reading jumps while moving harness | Intermittent wiring or connector issue |
| Cooling fans run constantly | ECM fail-safe response |
Common P0117 Diagnostic Mistakes
Assuming the Sensor Is Always Bad
The sensor is common, but shorted wiring or actual overheating can create the same code.
Ignoring the Cooling System
Always verify the engine is not truly overheating before focusing only on electrical diagnosis.
Replacing the Thermostat Without Testing
A thermostat may cause overheating, but it will not repair a shorted sensor circuit.
Confusing Circuit Voltage With Temperature
“Low input” means low electrical voltage, not low engine temperature.
Replacing the ECM Too Early
ECM failure is rare. Test the sensor, wiring, connector, and cooling system first.
Using Only the Dashboard Gauge
The gauge may use a different sensor or heavily filtered ECM data. Scan-tool readings provide more useful diagnostic information.
Vehicle-Specific P0117 Problems
Chevrolet and GMC
Common causes include:
- Failed ECT sensor
- Wiring damage near the thermostat housing
- Connector corrosion
- Actual overheating
- Coolant intrusion
Common models include:
- Silverado
- Sierra
- Tahoe
- Yukon
- Equinox
- Malibu
Ford
Common problems include:
- Cylinder Head Temperature sensor faults
- ECT wiring damage
- Connector issues
- Cooling fan or thermostat faults
Common models include:
- F-150
- Explorer
- Escape
- Fusion
- Mustang
Some Ford vehicles use a Cylinder Head Temperature sensor rather than a coolant-mounted sensor.
Toyota and Lexus
Typical causes include:
- Failed ECT sensor
- Shorted wiring
- Connector contamination
- Cooling-system overheating
Common models include:
- Camry
- Corolla
- Tacoma
- RAV4
- Highlander
- Lexus ES
- Lexus RX
Honda and Acura
Common problems include:
- Failed coolant temperature sensor
- Connector corrosion
- Wiring near the thermostat housing
- Overheating from low coolant
Common models include:
- Civic
- Accord
- CR-V
- Pilot
- Acura TL
- Acura MDX
Nissan and Infiniti
Common causes include:
- Shorted ECT sensor
- Wiring damage
- Connector corrosion
- Cooling-system faults
Common models include:
- Altima
- Maxima
- Rogue
- Frontier
- Pathfinder
- Infiniti G35
- Infiniti Q50
Hyundai and Kia
Typical causes include:
- ECT sensor failure
- Damaged wiring
- Connector contamination
- Cooling fan problems
- Low coolant
Subaru
Common problems include:
- Coolant sensor failure
- Wiring damage
- Air pockets after cooling-system service
- Actual overheating
BMW
Common causes include:
- Temperature sensor failure
- Wiring problems
- Electronic thermostat faults
- Water pump failure
- Cooling-system overheating
Volkswagen and Audi
Typical issues include:
- Failed coolant temperature sensor
- Internal dual-element sensor failure
- Connector corrosion
- Thermostat or water pump problems
P0117 Repair Costs
| Repair | Estimated Cost |
| Connector cleaning | $10–$75 |
| Coolant temperature sensor | $50–$250 |
| Wiring repair | $100–$400 |
| Connector replacement | $75–$300 |
| Thermostat replacement | $150–$500 |
| Cooling fan repair | $200–$900 |
| Water pump replacement | $400–$1,500 |
| Head gasket repair | $1,500–$4,000+ |
| ECM repair or replacement | $800–$2,000+ |
Related Trouble Codes
P0117 may appear with:
- P0115 – Engine Coolant Temperature Sensor Circuit Malfunction
- P0116 – Engine Coolant Temperature Sensor Range/Performance
- P0118 – Engine Coolant Temperature Sensor Circuit High Input
- P0125 – Insufficient Coolant Temperature for Closed-Loop Fuel Control
- P0128 – Coolant Temperature Below Thermostat Regulating Temperature
- P0217 – Engine Overtemperature Condition
- P2181 – Cooling System Performance
P0117 indicates a low-voltage signal, while P0118 indicates a high-voltage signal.
Frequently Asked Questions
Does P0117 mean the engine is overheating?
Not always.
It means the ECM is receiving a low-voltage signal that may indicate very high coolant temperature. The cause may be actual overheating, a failed sensor, or a shorted circuit.
Can I drive with P0117?
Only after confirming that the engine is not overheating.
If the temperature gauge is high, coolant is leaking, or steam is present, stop driving immediately.
Can a bad coolant temperature sensor cause P0117?
Yes.
An internally shorted coolant temperature sensor is one of the most common causes.
Can low coolant cause P0117?
Low coolant can contribute to actual overheating or unstable sensor readings, but P0117 is usually caused by an electrical low-voltage condition.
Can a bad thermostat cause P0117?
Yes, indirectly.
A thermostat stuck closed can cause genuine overheating, which may result in a low ECT sensor voltage. However, a thermostat does not typically cause an electrical circuit fault by itself.
Why are my cooling fans running constantly?
The ECM may activate the fans as a fail-safe if it believes the engine is overheating or cannot trust the coolant temperature signal.
Why does my scan tool show 300°F?
An extremely high reading on a cold engine usually indicates a shorted ECT sensor or signal wire.
Can P0117 cause hard starting?
Yes.
If the ECM believes the engine is already extremely hot during a cold start, it may reduce fuel enrichment too much, causing extended cranking or rough starting.
Will replacing the sensor fix P0117?
It will if the sensor is shorted internally.
It will not fix damaged wiring, connector corrosion, an actual overheating condition, or a short to ground elsewhere in the circuit.
Final Thoughts
The P0117 Engine Coolant Temperature Sensor Circuit Low Input code means the ECM is receiving a coolant temperature signal voltage that is lower than expected. This may indicate a shorted sensor, grounded signal wire, damaged connector, or an engine that is actually overheating.
The first step is always to verify the engine’s true temperature and check coolant level. If the engine is not overheating, compare live coolant temperature data with ambient temperature and inspect the sensor connector and wiring.
A scan-tool reading near 300°F on a cold engine strongly suggests an electrical fault. By diagnosing the sensor circuit before replacing parts, you can avoid unnecessary cooling-system repairs and determine whether P0117 is merely lying about the temperature—or warning you that the engine is genuinely trying to melt itself.
