P0117 — ECT Sensor Circuit Low Input

Quick answer

• What the P0117 code means: The Engine Coolant Temperature (ECT) sensor is sending a voltage signal to the Engine Control Module (ECM) that is lower than expected, indicating the engine's temperature is likely very cold or there is an electrical issue.
• Severity: Moderately severe. While you can often drive home or to a repair shop, prolonged driving can lead to poor fuel economy, increased emissions, and potential engine damage if the engine actually overheats and the gauge is not reflecting it.
• Most common cause: A faulty Engine Coolant Temperature (ECT) sensor or a problem with its wiring/connector.

What the P0117 code means

The P0117 diagnostic trouble code stands for "Engine Coolant Temperature (ECT) Sensor Circuit Low Input." This means that the Engine Control Module (ECM), which is your vehicle's main computer, has detected a voltage signal from the Engine Coolant Temperature sensor that is consistently lower than the normal operating range. The ECT sensor is a thermistor, meaning its electrical resistance changes with temperature. As the coolant temperature increases, the resistance decreases, and the voltage signal sent to the ECM also decreases. Conversely, as the coolant temperature decreases, the resistance increases, and the voltage signal increases. A "low input" suggests the ECM is receiving a voltage signal that indicates an extremely cold engine temperature, even if the engine is warm.

In plain English, your car's computer thinks the engine is much colder than it actually is, or there's an issue with the sensor or its wiring not sending the correct signal.

Common symptoms

• Check Engine Light illumination: This is almost always the first and most obvious symptom.
• Hard starting, especially when cold: The ECM may over-fuel the engine, assuming it's very cold, making it difficult to start.
• Poor fuel economy: The ECM will try to enrich the fuel mixture to compensate for what it believes is a constantly cold engine, leading to excessive fuel consumption.
• Black smoke from the exhaust: A rich fuel mixture can cause unburnt fuel to exit the exhaust, producing black smoke.
• Engine running rough or idling poorly: The incorrect fuel mixture can throw off engine performance.
• Engine overheating (in some cases): If the sensor is reporting an abnormally cold temperature, the cooling fans might not turn on when needed, or the temperature gauge might read inaccurately, leading to actual overheating without the driver being aware via the dash gauge.
• No heat from the HVAC system: If the ECM incorrectly believes the engine is cold, it might not command the heater blend door to open fully.
• Inaccurate temperature gauge reading: The dash gauge may read very low or not move at all, as it often uses data from the same sensor.

Common causes

1. Faulty Engine Coolant Temperature (ECT) sensor: This is the most common culprit. The internal components of the sensor can fail, causing it to send an incorrect voltage signal.
2. Wiring harness issues:
   • Damaged or shorted wiring: The wires leading to and from the ECT sensor can become frayed, cut, or shorted to ground, leading to a consistently low voltage signal.
   • Corroded or loose connector: Environmental factors (moisture, road salt) can corrode the sensor connector pins, increasing resistance and affecting the signal. A loose connection can also cause intermittent or consistent low voltage.
3. Low engine coolant level: While not a direct cause of a "low input" circuit issue, if the coolant level is so low that the sensor is not submerged in coolant, it won