If your Check Engine Light is on and your OBD-II scanner displays P0118, the Engine Control Module has detected an abnormally high-voltage signal from the Engine Coolant Temperature sensor circuit.
Despite the wording, P0118 does not usually mean the engine coolant is dangerously hot. In most cases, the ECM is receiving a signal that suggests the engine is impossibly cold—often around -40°F or -40°C—because the sensor circuit is open, unplugged, damaged, or experiencing excessive resistance.
The Engine Coolant Temperature sensor helps control fuel delivery, ignition timing, cooling fan operation, idle speed, emissions systems, and transmission behavior. When its signal becomes unreliable, engine performance and fuel economy can suffer.
Quick Answer
| Item | Information |
|---|---|
| Code | P0118 |
| Description | Engine Coolant Temperature Sensor Circuit High Input |
| Severity | ⭐⭐⭐☆☆ Moderate |
| Safe to Drive? | Usually for short distances |
| Most Common Causes | Unplugged sensor, damaged wiring, failed ECT sensor |
| Estimated Repair Cost | $20–$500 |
What Does the P0118 Code Mean?
Diagnostic Trouble Code P0118 means the ECM has detected voltage from the Engine Coolant Temperature sensor circuit that is higher than the acceptable range.
The ECT sensor is normally a temperature-sensitive resistor known as a thermistor. Its electrical resistance changes as coolant temperature changes.
Generally:
- Cold coolant creates higher resistance and higher signal voltage.
- Warm coolant creates lower resistance and lower signal voltage.
When the circuit voltage remains excessively high, the ECM may interpret the signal as an extremely cold engine.
Common reasons include:
- Unplugged ECT sensor
- Broken signal wire
- Poor electrical ground
- Corroded connector
- Failed coolant temperature sensor
- Open circuit
What Does the Engine Coolant Temperature Sensor Do?
The Engine Coolant Temperature sensor measures the temperature of the coolant circulating through the engine.
The ECM uses this information to control:
- Cold-start fuel enrichment
- Fuel injector pulse width
- Ignition timing
- Idle speed
- Radiator cooling fans
- Exhaust emissions systems
- Transmission shift scheduling
- Closed-loop fuel control
During a cold start, the engine needs additional fuel. Once the engine warms up, the ECM reduces enrichment and begins using normal operating strategies.
A faulty coolant temperature signal can make the ECM behave as though the engine is permanently cold.
Where Is the Coolant Temperature Sensor Located?
The ECT sensor is usually mounted in a coolant passage near one of the following areas:
- Cylinder head
- Thermostat housing
- Intake manifold
- Engine coolant outlet
- Upper radiator hose connection
Some vehicles use more than one coolant temperature sensor.
One sensor may supply information to the ECM, while another controls the dashboard temperature gauge. On other vehicles, one sensor performs both functions.
Always confirm the correct sensor using a service manual or wiring diagram before replacing parts. Otherwise, you may replace the gauge sensor and wonder why the code is still there. Automotive diagnostics: where confidence and guessing frequently arrive in the same toolbox.
Symptoms of a P0118 Code
P0118 symptoms vary depending on the vehicle and the ECM’s backup strategy.
Common symptoms include:
- Check Engine Light
- Hard starting
- Extended cranking
- Rough idle
- High idle speed
- Poor fuel economy
- Rich air/fuel mixture
- Black exhaust smoke
- Strong fuel smell
- Engine hesitation
- Cooling fans running constantly
- Cooling fans not operating correctly
- Failed emissions inspection
Some vehicles may appear to drive normally because the ECM substitutes a default coolant temperature value.
Why Does P0118 Sometimes Show -40 Degrees?
A scan tool reading of approximately -40°F or -40°C is a major diagnostic clue.
This usually indicates an open circuit caused by:
- Disconnected ECT sensor
- Broken wire
- Failed sensor
- Corroded connector
- Missing sensor ground
Negative 40 is commonly used because Fahrenheit and Celsius happen to meet at that temperature, giving automotive engineers one convenient universal way to say, “Yeah, that reading is nonsense.”
How Serious Is P0118?
Severity: ⭐⭐⭐☆☆ Moderate
P0118 usually does not mean immediate internal engine damage is occurring. However, inaccurate coolant temperature information can cause the ECM to deliver too much fuel or operate the cooling fans incorrectly.
Ignoring the code may result in:
- Poor fuel economy
- Carbon buildup
- Spark plug fouling
- Catalytic converter damage
- Increased exhaust emissions
- Hard starting
- Engine overheating if cooling fan operation is affected
The code should be diagnosed reasonably soon.
Can You Drive With P0118?
You can usually drive a short distance with P0118 if:
- The engine is not overheating
- Coolant level is normal
- The cooling fans operate
- The engine runs reasonably well
- There are no coolant leaks
Stop driving if:
- The temperature gauge enters the hot zone
- Steam appears from the engine compartment
- Coolant is leaking
- The engine misfires severely
- Black smoke becomes excessive
- The cooling fans fail to activate
- An overheating warning appears
Because the ECM may not know the actual coolant temperature, monitor the engine carefully.
Most Common Causes of P0118
| Cause | Likelihood |
| Unplugged ECT sensor | ⭐⭐⭐⭐⭐ |
| Failed coolant temperature sensor | ⭐⭐⭐⭐⭐ |
| Broken or open signal wire | ⭐⭐⭐⭐ |
| Corroded electrical connector | ⭐⭐⭐⭐ |
| Damaged sensor ground | ⭐⭐⭐ |
| Loose connector pins | ⭐⭐⭐ |
| Coolant contamination inside connector | ⭐⭐ |
| Faulty ECM connector | ⭐⭐ |
| Failed ECM | ⭐ |
Electrical faults are far more common than ECM failure.
Most Common Repairs
| Repair | Typical Cost |
| Reconnect ECT sensor | Free |
| Clean electrical connector | $10–$75 |
| Replace coolant temperature sensor | $50–$250 |
| Repair wiring | $100–$400 |
| Replace damaged connector | $75–$300 |
| Replace or repair ECM | $800–$2,000+ |
Repair costs vary by vehicle, labor rate, sensor location, and accessibility.
Tools You May Need
Diagnosing P0118 may require:
- OBD-II scanner
- Scan tool with live data
- Digital multimeter
- Infrared thermometer
- Wiring diagram
- Electrical contact cleaner
- Back-probe pins
- Basic hand tools
- Cooling-system pressure tester
- Service manual
A professional scan tool is helpful, but many P0118 problems can be diagnosed with live data, a multimeter, and a careful visual inspection.
Step-by-Step P0118 Diagnosis
Step 1: Check the Actual Engine Temperature
Before assuming the sensor is wrong, make sure the engine is not overheating.
Check:
- Dashboard temperature gauge
- Coolant level
- Cooling fan operation
- Visible coolant leaks
- Steam or coolant odor
Never remove the radiator cap from a hot engine. Pressurized coolant is not known for its patience or respect for human skin.
Step 2: Read Freeze-Frame Data
Use the scan tool to record:
- Coolant temperature
- Engine RPM
- Vehicle speed
- Intake air temperature
- Engine load
- Battery voltage
Freeze-frame data shows the conditions present when the code was stored.
If coolant temperature shows approximately -40 degrees, suspect an open circuit.
Step 3: Compare Coolant and Intake Temperatures
Allow the vehicle to sit overnight.
Before starting the engine, compare:
- Engine Coolant Temperature
- Intake Air Temperature
- Ambient air temperature
All three should be relatively close.
For example, if ambient temperature is 70°F and intake temperature is 72°F, but coolant temperature reads -40°F, the ECT circuit is clearly reporting incorrectly.
Step 4: Inspect the Sensor Connector
Locate the ECT sensor and check for:
- Disconnected plug
- Broken locking tab
- Bent terminals
- Green corrosion
- Coolant intrusion
- Loose pins
- Damaged insulation
Reconnect the sensor securely and monitor live data again.
A connector disturbed during thermostat, radiator, intake, or cylinder-head repairs is a common cause.
Step 5: Inspect the Wiring Harness
Follow the wiring from the sensor toward the ECM.
Look for:
- Chafed insulation
- Broken wires
- Melted wiring
- Rodent damage
- Harness tension
- Previous repairs
- Oil or coolant contamination
Pay special attention to areas where the harness contacts brackets, engine covers, or hot exhaust components.
Step 6: Perform a Connector Test
With the ignition on, disconnect the ECT sensor and observe the scan-tool reading.
Many systems will display an extremely low temperature with the sensor disconnected.
Next, follow the manufacturer’s diagnostic procedure for checking the signal and ground circuits. On some vehicles, briefly bridging the appropriate sensor circuit terminals may cause the scan-tool temperature reading to move toward an extremely hot value.
Do not randomly short terminals together without a wiring diagram. Electricity is already invisible; there is no need to make it mysterious too.
Step 7: Test Sensor Resistance
Remove or disconnect the sensor and measure resistance with a digital multimeter.
Sensor resistance should change smoothly as temperature changes.
Generally:
- Cold sensor: higher resistance
- Warm sensor: lower resistance
Compare the reading with the manufacturer’s temperature-versus-resistance chart.
A sensor showing infinite resistance is electrically open and likely defective.
Step 8: Check the Reference and Ground Circuits
Depending on the system design, verify:
- Reference voltage
- Sensor ground
- Signal continuity
- Resistance between sensor and ECM
- No short to battery voltage
High circuit voltage commonly results from an open signal circuit or missing ground.
Step 9: Verify the Repair
After repairing the problem:
- Reconnect all components.
- Clear the code.
- Start the engine from cold.
- Monitor coolant temperature.
- Confirm the reading rises smoothly as the engine warms.
- Verify cooling fan operation.
- Perform a road test.
- Rescan for pending or stored codes.
The coolant reading should not jump erratically or remain fixed at an unrealistic temperature.
Fuel-Trim Clues for P0118
When the ECM believes the engine is extremely cold, it may command additional fuel.
Possible scan-tool clues include:
- Negative fuel trims
- Rich exhaust readings
- Increased injector pulse width
- Reduced fuel economy
- Fuel smell
- Black smoke
However, many modern vehicles use a substitute temperature value when the circuit fails, so fuel-trim behavior may vary.
Common P0118 Diagnostic Mistakes
Replacing the Sensor Without Inspecting the Connector
A new sensor cannot repair a broken wire, corroded terminal, or connector that was never plugged back in.
Assuming P0118 Means the Engine Is Overheating
“Circuit high input” refers to electrical voltage, not necessarily high coolant temperature.
Ignoring Live Data
The coolant temperature reading often reveals the problem immediately.
Replacing the Thermostat First
A thermostat problem usually causes codes such as P0128, not an electrical circuit-high code like P0118.
Assuming the ECM Is Bad
ECM failure is possible but rare. Inspect the sensor, connector, wiring, reference voltage, and ground first.
Confusing the Gauge Sensor With the ECM Sensor
Some vehicles use separate temperature sensors. Replacing the wrong one accomplishes little beyond making the parts store slightly richer.
Vehicle-Specific P0118 Problems
Chevrolet and GMC
Common causes include:
- Failed ECT sensor
- Corroded sensor connector
- Wiring damage near the thermostat housing
- Coolant intrusion into the connector
Common models include:
- Silverado
- Sierra
- Equinox
- Malibu
- Tahoe
- Yukon
Ford
Common problems include:
- Cylinder Head Temperature sensor faults on applicable engines
- Damaged wiring
- Connector corrosion
- Harness damage near the engine
Common models include:
- F-150
- Escape
- Explorer
- Fusion
- Mustang
Some Ford engines use a Cylinder Head Temperature sensor rather than a traditional coolant-mounted ECT sensor.
Toyota and Lexus
Common causes include:
- Failed ECT sensor
- Connector damage after cooling-system work
- Open signal circuit
- Corrosion inside the connector
Common models include:
- Camry
- Corolla
- Tacoma
- RAV4
- Highlander
- Lexus ES
- Lexus RX
Honda and Acura
Common causes include:
- ECT sensor failure
- Damaged connector
- Wiring near the thermostat housing
- Low-quality replacement sensors
Common models include:
- Civic
- Accord
- CR-V
- Pilot
- Acura TL
- Acura MDX
Nissan and Infiniti
Common issues include:
- Failed coolant temperature sensor
- Connector corrosion
- Damaged wiring
- Poor terminal tension
Common models include:
- Altima
- Maxima
- Rogue
- Frontier
- Pathfinder
- Infiniti G35
- Infiniti Q50
Hyundai and Kia
Typical causes include:
- Faulty ECT sensor
- Wiring damage
- Connector contamination
- Cooling-system repair errors
Subaru
Common problems include:
- Coolant sensor failure
- Wiring damage
- Corroded connectors
- Incorrect readings after engine or cooling-system repairs
BMW
Common causes include:
- Coolant temperature sensor failure
- Damaged wiring
- Coolant contamination in electrical connectors
- Integrated temperature sensor faults on applicable cooling components
Volkswagen and Audi
Typical issues include:
- Failed coolant temperature sensor
- Internal dual-element sensor failure
- Connector corrosion
- Wiring damage
On many Volkswagen and Audi vehicles, coolant temperature sensors have historically been frequent failure points.
P0118 Repair Costs
| Repair | Estimated Cost |
| Sensor reconnection | Free |
| Electrical connector cleaning | $10–$75 |
| Coolant temperature sensor replacement | $50–$250 |
| Wiring repair | $100–$400 |
| Connector replacement | $75–$300 |
| Coolant service | $100–$250 |
| ECM diagnosis or replacement | $800–$2,000+ |
Some ECT sensors are easy to reach, while others require removing intake components or draining part of the cooling system.
Related Trouble Codes
P0118 may appear with:
- P0115 – Engine Coolant Temperature Sensor Circuit Malfunction
- P0116 – Engine Coolant Temperature Sensor Range/Performance
- P0117 – Engine Coolant Temperature Sensor Circuit Low 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
Can low coolant cause P0118?
Low coolant does not usually cause a circuit-high code directly.
However, coolant loss can expose the sensor or create temperature irregularities. Always inspect coolant level and leaks during diagnosis.
Can a bad thermostat cause P0118?
Usually not.
A stuck-open thermostat more commonly causes P0128. P0118 generally indicates an electrical problem involving the sensor circuit.
Can I drive with P0118?
Short-distance driving is usually possible if the engine is not overheating and the cooling fans operate normally. The problem should still be repaired soon.
Will replacing the coolant temperature sensor fix P0118?
It will if the sensor itself has failed.
If the problem is damaged wiring, corrosion, a loose connector, or an open ground circuit, replacing the sensor will not solve it.
Why are my radiator fans running constantly?
Many ECMs activate the cooling fans as a fail-safe when the coolant temperature signal becomes unreliable.
The computer would rather run the fans unnecessarily than assume everything is fine while the engine quietly cooks itself.
Can P0118 cause hard starting?
Yes.
If the ECM believes the engine is extremely cold, it may add too much fuel during startup, causing rich operation, extended cranking, or rough running.
Can P0118 cause black smoke?
Yes.
An incorrect cold temperature signal can cause excessive fuel delivery, potentially producing black exhaust smoke and a strong fuel odor.
Does P0118 mean the coolant is too hot?
No.
The phrase “circuit high input” refers to high electrical signal voltage. It commonly causes the ECM to interpret the engine as extremely cold.
Can P0118 damage the catalytic converter?
Potentially.
If the incorrect coolant reading causes the engine to run excessively rich for an extended period, unburned fuel and higher emissions can damage the catalytic converter.
Final Thoughts
The P0118 Engine Coolant Temperature Sensor Circuit High Input code is usually caused by an electrical issue rather than an overheating engine. An unplugged sensor, failed ECT sensor, broken wire, corroded connector, or missing sensor ground can force the ECM to interpret coolant temperature as approximately -40 degrees.
Start diagnosis by checking coolant level and actual engine temperature, then inspect scan-tool data. If the reported coolant temperature is unrealistic, examine the sensor connector and wiring before replacing parts.
In many cases, P0118 can be repaired with a replacement sensor, connector repair, or corrected wiring fault. Proper diagnosis restores accurate fuel control, cooling fan operation, fuel economy, and engine performance—without firing the parts cannon at everything near the thermostat housing.
