If your Honda Civic’s check engine light is on and the code reader shows P0118, you’re likely looking for a clear explanation. What is P0118 in Honda Civic? It’s an OBD-II diagnostic trouble code that means your car’s computer sees a problem with the Engine Coolant Temperature (ECT) sensor circuit, specifically that the signal voltage is too high, indicating the engine is too cold when it’s actually not.
This sensor is a small but critical part. It tells the engine control module (ECM) how hot or cold the engine is. That data influences fuel mixture, ignition timing, idle speed, and even the operation of the radiator fan. A faulty reading from this sensor can cause real drivability issues and affect your car’s fuel economy.
What Is P0118 In Honda Civic?
Let’s break down the technical definition. Code P0118 stands for “Engine Coolant Temperature Sensor 1 Circuit High.” In simpler terms, your Civic’s main computer (the ECM) is reading a signal from the ECT sensor that is outside the expected range—specifically, it’s reading a voltage that corresponds to an extremely cold temperature, like -40°F, even when the engine is warm or at operating temperature.
This creates a conflict. The ECM knows the engine has been running, but the sensor says it’s freezing. Because the computer trusts its sensors, it will make adjustments based on this faulty cold reading, which can lead to several performance problems.
What Does the Engine Coolant Temperature Sensor Do?
The ECT sensor is typically a two-wire sensor screwed into the engine’s thermostat housing or a coolant passage. It’s essentially a thermistor, meaning its electrical resistance changes with temperature.
- When the coolant is cold, the sensor’s resistance is high.
- When the coolant heats up, the resistance drops.
The ECM sends a reference voltage (usually 5 volts) through the sensor and measures the voltage drop. A high resistance (cold engine) returns a high signal voltage. A low resistance (hot engine) returns a low signal voltage. The P0118 code triggers when that signal voltage stays high, suggesting a permanently cold condition.
Common Symptoms of a P0118 Code
You might notice these signs before or along with the check engine light:
- A check engine light (often with a solid glow, but sometimes flashing if misfires occur).
- Poor fuel economy, as the ECM enriches the fuel mixture for a “cold” engine.
- Hard starting when the engine is actually warm.
- Rough idle or unstable engine speed.
- The engine may overheat because the radiator fan doesn’t get the correct signal to turn on.
- Black smoke from the exhaust due to an overly rich fuel mixture.
Main Causes of the P0118 Code in Your Honda Civic
Several issues can lead to this high voltage reading. Here are the most common culprits, starting with the most likely.
1. A Faulty ECT Sensor
This is the most common cause. The sensor’s internal resistor can fail, causing it to get stuck reporting a high-resistance (cold) state. Sensors can also fail due to age, corrosion, or physical damage.
2. Problems in the Wiring or Connectors
The circuit from the sensor to the ECM can be the issue. Look for:
- An open or short in the sensor’s signal wire.
- A short to power in the signal wire (touching a live 12V wire).
- A poor connection, corrosion, or damage at the sensor connector.
- Damage to the wiring harness from heat, rodents, or chafing.
3. Issues with the Reference Voltage or Ground Circuit
Less commonly, the problem might not be with the sensor itself but with the power or ground it recieves. A poor ground connection for the sensor can disrupt the circuit and cause a high voltage reading.
4. A Failing Engine Control Module (ECM)
This is the least common cause. It’s possible, but very rare, for the ECM itself to have an internal fault that prevents it from reading the sensor signal correctly. Always check the simpler, cheaper causes first.
How to Diagnose and Fix P0118: A Step-by-Step Guide
Safety first: Always let the engine cool completely before working near the cooling system to avoid burns. Disconnect the negative battery terminal before touching electrical connectors.
Tools you might need: A basic socket set, a digital multimeter, a scan tool or code reader, and possibly a new ECT sensor (get an OEM or high-quality aftermarket part for best results).
Step 1: Confirm the Code and Check Live Data
Clear the P0118 code with your scan tool. Start the engine and let it idle. Use your scan tool to monitor the live data parameter for “Engine Coolant Temperature” or “ECT.”
- If the sensor is working, the temperature reading should start low and gradually rise to about 190–220°F as the engine warms up.
- If you see a reading stuck at an implausibly cold value (e.g., -40°F or -40°C) while the engine is warm, this confirms the issue.
Step 2: Perform a Visual Inspection
Locate the ECT sensor on your Civic’s engine (consult your owner’s manual or a repair guide for its exact location). Carefully inspect:
- The sensor’s connector for any cracks, corrosion, or bent pins.
- The wiring harness for any obvious damage, burns, or cuts.
- Make sure the sensor is properly seated and not leaking coolant.
Step 3: Test the ECT Sensor with a Multimeter
You’ll need to test both resistance and voltage.
- With the engine cool and the connector disconnected, set your multimeter to measure resistance (Ohms).
- Touch the meter’s probes to the two terminals of the sensor itself.
- Compare your reading to the manufacturer’s specifications (usually available online). A cold engine should have a high resistance (several thousand ohms), and a warm engine should have a low resistance (a few hundred ohms). If the resistance is extremely high or infinite when the engine is warm, the sensor is bad.
Next, test for voltage. Reconnect the sensor connector. Carefully back-probe the signal wire with the multimeter’s positive probe and ground the negative probe. With the key in the “ON” position (engine off), you should see about 5 volts on a cold engine, dropping as it warms. A constant 5 volts or very high voltage when warm indicates a circuit or sensor problem.
Step 4: Check the Wiring Harness
If the sensor tests okay, the wiring is next. Disconnect the connector at both the sensor and the ECM. Check for continuity in the signal wire between the two ends. Also check that the signal wire is not shorted to power or ground. Repair any damaged wires you find.
Step 5: Replace the Faulty Component
Once you’ve identified the root cause:
- If it’s the sensor, drain a small amount of coolant below the sensor level (or be ready to plug the hole quickly). Unscrew the old sensor, install the new one with a proper seal (often a new copper washer), and reconnect it.
- If it’s wiring, repair or replace the damaged section using proper solder and heat shrink tubing.
Reconnect the battery, clear the code, and start the engine. Verify that the check engine light stays off and that the live data from the new sensor shows a normal, rising temperature.
What Happens If You Ignore the P0118 Code?
Driving with a P0118 code for an extended period is not recommended. The consequences can include:
- Significantly reduced gas mileage, costing you more at the pump.
- Increased carbon buildup on spark plugs and inside the engine from the rich fuel mixture.
- Potential for engine overheating and severe damage if the radiator fan circuit is affected.
- General poor performance and drivability, making the car unpleasant to drive.
It’s best to adress the issue promptly to avoid more expensive repairs down the road.
FAQs About Honda Civic P0118
Can I drive my Civic with a P0118 code?
You might be able to drive it for a short distance to a repair shop, but it’s not advisable for daily use. Performance and fuel economy will suffer, and there’s a risk of overheating.
How much does it cost to fix a P0118 code?
The cost varies. If it’s just the sensor, the part is usually between $20 and $80. If you do it yourself, that’s the total cost. A mechanic might charge 1-2 hours of labor, bringing the total to $150-$300. Wiring repairs can increase the cost depending on complexity.
Is the ECT sensor the same as the coolant temperature gauge sender?
No, they are often separate sensors. Your Civic has an ECT sensor for the ECM (which triggers P0118) and a separate sender unit for the temperature gauge on your dashboard. One can fail without affecting the other.
Could a low coolant level cause P0118?
Not directly. The sensor needs to be immersed in coolant to read accurately. If coolant is low, the sensor might read air temperature instead of coolant temp, which could cause erratic readings, but it typically wouldn’t cause a consistent “circuit high” P0118 code. Still, always check your coolant level.
Will a bad thermostat cause a P0118?
A stuck thermostat usually causes overheating or the engine running too cool, which would change the actual temperature reading from a working sensor. It wouldn’t typically cause the specific electrical “circuit high” fault that defines P0118. The code points to an electrical fault in the sensor or its circuit.
Final Thoughts on Code P0118
While a P0118 code in your Honda Civic can be an annoyance, it’s often a straightforward fix. The problem usually lies with the Engine Coolant Temperature sensor itself or its wiring. By following a logical diagnosis process—checking live data, doing a visual inspection, and performing some simple electrical tests—you can identify the root cause without guesswork.
Addressing this code quickly restores your Civic’s performance and fuel efficiency. It also prevents potential secondary issues like overheating. For most DIY enthusiasts, replacing the ECT sensor is a very manageable job. If your not comfortable with the electrical testing, having a trusted mechanic perform the diagnosis can still be a cost-effective solution to get your Civic running smoothly again.