P0117 OBD2 Code Guide
P0117 is a sensors diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
Engine Coolant Temperature (ECT) Circuit Low Input
Code P0117 indicates that the Engine Coolant Temperature (ECT) sensor circuit is reporting a voltage lower than expected to the Powertrain Control Module (PCM). Since the ECT sensor is typically a Negative Temperature Coefficient (NTC) thermistor, a low voltage signifies very low resistance, which the PCM interprets as an excessively high engine coolant temperature. This false high-temperature reading can cause the PCM to incorrectly enrich the fuel mixture, retard ignition timing, and activate cooling fans prematurely, leading to symptoms like poor fuel economy, black smoke from the exhaust, rough idle, or difficulty starting. It could also mask an actual overheating condition if the sensor is shorted.
Code verification standard
P0117 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0117 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0117 diagnostic path
- 1Visual Inspection and Scan Tool Data Review: Begin by visually inspecting the ECT sensor connector and wiring harness for any signs of damage, corrosion, or shorts to ground. Check for proper seating of the connector. Use a scan tool to observe the live ECT sensor data. If the reading is extremely high (e.g., 250°F / 120°C or more) when the engine is cold, it strongly suggests a P0117 condition. Check for other related DTCs.
- 2ECT Sensor Resistance Test: Disconnect the ECT sensor and measure its resistance using a multimeter. Compare the reading to the manufacturer's specifications for the ambient temperature. As an NTC thermistor, resistance should decrease as temperature increases. A very low or near-zero resistance reading, especially when cold, indicates an internal short within the sensor.
- 3Wiring Integrity Check: With the sensor disconnected, check the ECT sensor circuit for continuity between the sensor connector and the PCM connector for both the signal wire and the ground wire. Also, check for any shorts to ground or shorts to voltage on both wires. Verify the 5-volt reference voltage at the sensor connector from the PCM.
- 4PCM Input Voltage Verification: Reconnect the ECT sensor. Using a multimeter, back-probe the signal wire at the ECT sensor connector (or at the PCM connector, if accessible) and measure the voltage. Compare this voltage to the expected values based on the engine's actual temperature. A consistently low voltage (e.g., near 0V) confirms the PCM is receiving a low input, indicating a problem in the sensor or wiring.
Verify before repair
- - Replace the faulty Engine Coolant Temperature (ECT) sensor.
- - Repair or replace damaged, corroded, or shorted wiring in the ECT sensor circuit.
- - Repair or replace the ECT sensor connector if it's corroded or damaged.
- - Replace the Powertrain Control Module (PCM) if all other components and wiring test good (less common).
Vehicle-specific pages to check next
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