P0118
UniversalEngine Coolant Temperature Circuit High Input
Medium SeverityThe P0118 code indicates that the vehicle's computer is receiving an unusually high voltage signal from the Engine Coolant Temperature (ECT) sensor. This often means the sensor circuit is open or has high resistance, leading the computer to believe the engine coolant is extremely cold, which can negatively impact fuel delivery and engine performance.
⚠️ Before You Begin (Safety First)
Always ensure the engine is cool before working with the cooling system to avoid severe burns from hot coolant. Disconnect the negative battery terminal before performing any electrical work to prevent short circuits or electrical shocks.
Common Symptoms
- Check Engine Light illuminated
- Engine running rich (excessive fuel)
- Poor fuel economy
- Hard starting, especially when cold
- Black smoke from exhaust
- Incorrect temperature gauge reading
- Engine may overheat (due to incorrect fan operation)
Common Causes
- Faulty Engine Coolant Temperature (ECT) sensor
- Open or short circuit in the ECT sensor wiring
- Corroded or loose electrical connections at the ECT sensor or PCM
- Faulty PCM (less common)
Frequently reported on: All Makes
Diagnostic Steps
- Inspect the ECT sensor wiring and connector for any visible damage, corrosion, or loose connections.
- With the engine cool, locate the ECT sensor (usually near the thermostat housing or cylinder head).
- Disconnect the electrical connector from the sensor.
- Using a multimeter, test for voltage at the connector with the ignition on to check the circuit integrity. You can also test the sensor's resistance.
- If the sensor is suspected to be faulty, carefully drain a small amount of coolant (if necessary), remove the old sensor, and install a new one.
- Reconnect the electrical connector, refill coolant, and clear the code with an OBD-II scanner.
👨🔧 When to Call a Professional
If after checking the wiring and replacing the ECT sensor, the code persists, or if other complex electrical issues are present, it's best to consult a professional technician for advanced diagnostics.
Commonly Asked Questions
Can I drive with a P0118 code?
While you might be able to drive, it's not recommended for long periods as it can lead to poor fuel economy, increased emissions, and potentially engine damage if the engine runs too rich or overheats due to incorrect coolant temperature readings.
Is the ECT sensor the same as the coolant temperature sender for the dashboard gauge?
Not always. Many modern vehicles use one ECT sensor for the PCM and a separate sender for the dashboard gauge, though some combine these functions into a single sensor.
How can I test the ECT sensor myself?
You can test its resistance using a multimeter while monitoring its change as the engine warms up, comparing readings to your vehicle's service manual specifications. You can also check for proper reference voltage and ground at the sensor connector.
What if replacing the ECT sensor doesn't fix the P0118 code?
If a new sensor doesn't resolve the issue, the problem likely lies in the wiring harness, such as an open circuit, a short to voltage, or high resistance. Less commonly, the Powertrain Control Module (PCM) itself could be faulty.
Does low coolant cause a P0118 code?
Low coolant itself typically wouldn't cause a 'circuit high input' directly, as this usually signifies an electrical fault in the sensor or wiring. However, if the sensor is exposed to air instead of coolant due to a very low level, it might read inaccurately, but a 'circuit high input' is more indicative of an open electrical circuit.