P0118 — Engine Coolant Temperature Sensor Circuit 1 High Input

powertrain (engine/transmission)

Is it safe to drive?

Caution P0118 can cause incorrect fuel enrichment, difficult cold starts, and excessive cooling-fan operation. Drive cautiously to a repair location if the engine runs normally, but stop safely if the temperature gauge shows heat, the coolant warning appears, or the engine actually overheats.

Typical symptoms

The check-engine light reports P0118
The temperature gauge reads unusually high, sometimes while the engine is still cold
The cooling fans operate continuously or at a higher-than-normal speed
Cold starting is difficult because the engine control module richens the fuel mixture
Fuel economy or idle quality may deteriorate while P0118 remains active
The engine may overheat if the false temperature reading is accompanied by an actual cooling-system fault

Common causes

An open or high-resistance circuit in the engine coolant temperature sensor signal wire
A short to voltage in the sensor signal, reference, or ground wiring
A failed engine coolant temperature sensor with an internally open or high-resistance element
Corroded terminals, a loose connector, or damaged wiring at the engine coolant temperature sensor
An engine control module reference-voltage or ground fault involving sensor circuit 1

How to diagnose with an OBD scanner

Confirm P0118 with an OBD2 scanner Connect AIScanAuto, an ELM327 adapter, or another OBD2 scanner and review stored, pending, and freeze-frame data. Confirm that P0118 is active and check whether the reported engine coolant temperature is implausibly high.
Inspect the sensor and connector With the engine cold, locate engine coolant temperature sensor circuit 1 using the vehicle wiring diagram. Inspect the connector for pushed-back pins, corrosion, coolant contamination, and wiring damaged by heat or chafing.
Test the complete circuit Disconnect the sensor and use a wiring diagram to test for continuity and shorts. An open signal circuit or unintended voltage can create the high input identified by P0118. Also verify the sensor ground and reference circuits where applicable.
Measure the sensor resistance Measure the engine coolant temperature sensor resistance and compare it with the manufacturer specification for the measured temperature. The resistance should normally decrease as coolant temperature rises; an open or out-of-range sensor supports the P0118 diagnosis.
Repair, clear, and verify Repair the damaged wiring, clean and secure the connector, or replace the sensor as testing indicates. Clear P0118 with the scanner, then verify the temperature reading, cooling-fan operation, and whether the code returns. Repeated P0118 with correct wiring and sensor readings requires engine control module diagnosis.

Approximate repair cost

Independent-shop repairs for P0118 typically run about $100-$400 for diagnosis and a common sensor, connector, or wiring repair, depending on vehicle accessibility and labor time. These USD figures are estimates and vary by region; obtain a local quote before authorizing work.

Common questions

What does P0118 mean on an OBD2 scanner?

P0118 means the engine control module is receiving an excessively high voltage from engine coolant temperature sensor circuit 1, often because the circuit is open or the sensor has failed.

Can I drive with P0118 stored?

You can cautiously drive to a repair shop if the engine runs normally and the temperature gauge is reasonable, because P0118 can cause a rich mixture and continuous fan operation. Stop immediately if overheating or a high-temperature indication occurs.

Does P0118 always mean the coolant temperature sensor is bad?

No. P0118 can also result from an open wire, a short to voltage, a corroded connector, or a sensor-circuit ground or reference problem, so circuit testing should precede replacing the sensor.

Why is my temperature gauge high even when the engine is cold?

A high-input fault in engine coolant temperature sensor circuit 1 can make the gauge or engine control module believe the engine is extremely hot. Verify the circuit because P0118 may be reporting an electrical fault rather than actual overheating.

When your check engine light comes on and a scan tool pulls code P0118, the engine control unit is telling you it has spotted an abnormal voltage condition in the engine coolant temperature sensor circuit. Specifically, the signal the module is receiving sits higher than it should—often near battery voltage—while the engine is running. This is not a vague "something might be wrong" message; the ECU has compared live sensor data against its programmed limits, confirmed the reading is out of range, and logged the fault.

The engine coolant temperature sensor plays a straightforward but critical role. It is essentially a variable resistor that tells the control module how hot the engine is by changing its electrical resistance as coolant temperature changes. When coolant is cold, the sensor resistance is high and the voltage signal is low. As the engine warms up, resistance drops and voltage rises. The ECU expects to see a predictable voltage range that corresponds to the actual coolant temperature. When P0118 appears, the module is seeing the opposite of what makes sense—a signal voltage that is too high, as if the sensor circuit is being pulled up toward power instead of ground. This reading tells the computer the engine is far colder than it actually is, which throws off fuel injection timing, idle speed, cooling fan operation, and cabin heating.

Setting P0118 typically requires the fault to be present during multiple drive cycles, which is why the ECU also saves a freeze frame snapshot. That snapshot records engine RPM, load, coolant temperature, and vehicle speed at the exact moment the problem occurred. Technicians use that data to replicate the driving conditions and verify the repair.

Common Symptoms of P0118

Check Engine Light illuminated on the dashboard
Temperature gauge reading abnormally low or never climbing to normal operating range
Weak or absent cabin heat output
Cooling fans running at full speed more often than they should
Noticeable increase in fuel consumption due to the ECU enrichmenting the mixture based on a false cold reading
Overheating warnings in more severe cases where the actual engine temperature has risen while the sensor tells a different story

What Causes P0118?

Failed engine coolant temperature sensor pulling the signal line high
Wiring in the sensor circuit shorted directly to battery positive voltage
Broken or disconnected ground return for the sensor circuit
Low coolant level or trapped air in the cooling system disrupting temperature readings
Malfunctioning cooling fan motor or fan control relay
Faulty thermostat stuck in the open or closed position
Water pump or radiator problems affecting coolant circulation
General wiring damage such as chafing, corrosion, or loose connectors in the sensor harness

How to Diagnose P0118

Plug in an OBD2 scanner, retrieve P0118, and record the freeze frame data before clearing anything—this snapshot guides your entire diagnosis.
Look for any other stored trouble codes. Related codes often point to the underlying cause, and checking manufacturer TSBs for your specific make and model may reveal known issues or software updates.
Inspect the engine coolant temperature sensor wiring and connector visually. Look for damaged insulation, bent pins, corrosion buildup, and any signs that the harness has been rubbing against hot or moving parts.
With the sensor unplugged, probe the harness side of the connector with a multimeter. If you still read high voltage (near battery level) with the sensor disconnected, the wiring itself is shorted to power. If the voltage drops to normal range, the sensor has failed internally.
Monitor live coolant temperature data on the scanner during a warm-up cycle. A healthy signal climbs steadily until the thermostat opens and the temperature stabilizes. Erratic or frozen readings confirm sensor or circuit problems.
Verify coolant level and physically check the upper and lower radiator hoses for proper heat progression, which confirms the thermostat is functioning correctly.
After any repairs, clear the code and complete a drive cycle that matches the conditions captured in the freeze frame. The code should not return.

Repair Options and Costs

If testing confirms the engine coolant temperature sensor itself is faulty, replacement is the straightforward fix:

Replacement engine coolant temperature sensor: parts and labor typically run $100–$300

When the sensor checks out but the problem persists, other cooling system components may be at fault:

Thermostat replacement: $150–$400
Cooling system repair (water pump, radiator, fan motor or fan relay): $300–$1,200

How Much Does Fixing P0118 Cost?

The total bill hinges entirely on which part has failed. Most often the culprit is the inexpensive sensor itself, bringing the total toward the lower end of the range. Labor rates at independent shops average $75–$110 per hour, so a straightforward sensor swap with diagnosis usually lands around $150–$250 total. The worst-case scenario—internal engine cooling components like a water pump or radiator requiring coolant flush and hardware replacement—can push costs toward $1,200. Skipping a proper diagnosis and replacing parts randomly rarely saves money; spending time with a multimeter and scanner first almost always does.

Can You Drive with P0118?

Drive with caution. The ECU is operating on incorrect temperature data, which means the cooling fans may run unnecessarily, fuel consumption rises, and cabin heating suffers. A stuck-open thermostat mainly costs you money at the pump. However, if the engine is actually running hot while the gauge or sensor tells a different story, you risk serious damage—blown head gaskets, warped heads, or a cracked block. At the first sign of actual overheating (steam, coolant loss, gauge needle in the red), stop immediately and let the engine cool before investigating further.