DTC P0117: OBD-II Code – Engine Coolant Temperature Sensor Circuit Low Input

Understanding the OBD-II DTC P0117 Code

The Dtc Obdii Dtc P0117 code is a common diagnostic trouble code (DTC) in modern vehicles equipped with OBD-II systems. This code, which is generic and can apply across various makes and models from 1996 onwards (including Honda, Toyota, Volkswagen, Mazda, Ford, and BMW), signals an issue within the Engine Coolant Temperature (ECT) sensor circuit. While the code is broadly applicable, specific diagnostic and repair procedures may differ depending on the vehicle’s manufacturer and model.

The ECT sensor, a crucial component in your vehicle’s engine management system, is essentially a thermistor. It’s strategically positioned in the engine block or a coolant passage to accurately measure the engine coolant temperature. This sensor, typically a two-wire device, operates by changing its electrical resistance in response to temperature fluctuations of the coolant. One wire receives a 5-volt reference signal from the Powertrain Control Module (PCM), while the other provides a ground, also managed by the PCM.

As engine coolant temperature varies, the ECT sensor’s resistance changes accordingly. When the engine is cold, the resistance is high, and conversely, when the engine warms up, the resistance decreases. The P0117 code is triggered when the PCM detects a signal voltage from the ECT sensor that is lower than the expected normal operating range. This “low input” condition indicates to the PCM that the coolant temperature signal is abnormally low, potentially suggesting an excessively hot engine condition or a fault in the sensor circuit.

Common Symptoms Associated with DTC P0117

When the P0117 code is present, you may observe several symptoms affecting your vehicle’s performance and efficiency. These symptoms can range from minor inconveniences to more significant drivability issues:

• Malfunction Indicator Lamp (MIL) Illumination: The most immediate sign is the illumination of the check engine light on your dashboard. This is the vehicle’s way of alerting you to a detected problem within the engine management system.
• Reduced Fuel Economy: An inaccurate coolant temperature reading can lead the PCM to miscalculate fuel delivery, often resulting in a richer fuel mixture and decreased fuel efficiency.
• Poor Drivability: Engine performance may suffer. This can manifest as hesitation, stumbling, or a general lack of responsiveness, especially during acceleration.
• Rough Engine Running and Black Smoke: In some cases, an excessively rich fuel mixture caused by incorrect temperature readings can lead to the engine running roughly and even producing black smoke from the exhaust pipe, indicating unburnt fuel.
• Idling Problems: The engine may struggle to maintain a stable idle. It might idle erratically, too low, or even stall.
• Starting Issues: The vehicle might start and then immediately die, or it may be hard to start, especially in cold conditions, as the PCM relies on the ECT sensor for proper cold start fueling.

Potential Causes of the P0117 Code

Several factors can lead to the P0117 DTC. Identifying the root cause is crucial for effective repair. Here are the common culprits:

• Faulty ECT Sensor: The sensor itself might be defective. Thermistors can fail over time due to thermal stress, electrical issues, or physical damage.
• Short to Ground in ECT Signal Circuit: The wiring carrying the signal from the ECT sensor to the PCM may have a short to ground. This can occur due to damaged insulation, chafing, or rodent damage.
• Connector Issues: Faulty or damaged connectors at the ECT sensor or the PCM can disrupt the electrical signal. Corrosion, loose pins, or physical damage to the connectors can all cause problems.
• Wiring Harness Damage: The wiring harness containing the ECT sensor circuit may be damaged. This could involve cuts, breaks, or internal wire damage that isn’t immediately visible.
• Loose Terminals: Loose terminals at either the ECT sensor or the PCM can result in intermittent or no signal, triggering the P0117 code.
• Overheated Engine (Possible but Less Likely Direct Cause): While the code indicates a “low input” suggesting high temperature, an actual engine overheat could theoretically contribute, although it’s more likely the sensor or wiring is misreporting a normal or cold temperature as excessively hot due to the circuit fault. Address any overheating issues first, but focus on the sensor and wiring for P0117.
• PCM Failure (Rare): In less frequent cases, the PCM itself might be faulty. However, PCM failure is usually diagnosed after ruling out all other potential causes, as it is a more complex and expensive component.

Diagnosing and Resolving the P0117 DTC

Addressing the P0117 code requires a systematic diagnostic approach. Start by considering if the engine has actually overheated recently. If so, address the overheating issue first before proceeding with diagnosing the P0117 code. Assuming no overheat, follow these steps:

1. Initial Scan Tool Check: Using an OBD-II scan tool, read the ECT sensor value with the key on and engine off (KOEO). On a cold engine, the ECT reading should be reasonably close to the Intake Air Temperature (IAT) sensor reading. A significant discrepancy suggests a problem. If the ECT reading is implausibly high (e.g., above 260°F on a cold engine), proceed to the next step.

2. Sensor Unplug Test: Disconnect the ECT sensor. This should force the ECT reading on the scan tool to drop to an extremely low value (e.g., -30°F or lower).

   • If the reading drops: This indicates the sensor is likely faulty and internally shorted, causing the “low input” signal. Replace the ECT sensor.
   • If the reading does not change: This suggests a short to ground in the ECT signal circuit or potentially a PCM issue. Proceed to wiring checks.

3. Wiring Inspection for Short to Ground: Check the ECT sensor signal wire for a short to ground. Inspect the wiring harness for any signs of damage, chafing, or melted insulation. Pay close attention to areas where the harness might rub against metal components or engine parts. Repair any damaged wiring as needed. Also, check if the two wires to the ECT sensor are shorted together.

4. PCM Signal Wire Voltage Check: If no wiring issues are found, check for voltage output from the PCM on the ECT signal wire pin at the PCM connector (with the ECT sensor disconnected and key ON). You should expect to see approximately 5 volts.

   • No or low voltage: This could indicate a PCM fault, but consider a “temporary short” scenario first.

5. Check for 5-Volt Reference Short (Shared Circuit Issue): On some vehicles, the 5-volt reference signal is shared among multiple sensors. A short in another sensor on this shared circuit can pull down the 5-volt reference, affecting the ECT sensor reading and potentially triggering P0117 along with other sensor codes. If you suspect this, systematically disconnect each sensor that shares the 5-volt reference circuit, one at a time, while monitoring the 5-volt reference voltage at the PCM. If the 5-volt reference voltage returns to normal after disconnecting a specific sensor, that sensor is likely shorted and needs replacement.

6. Intermittent Fault Testing (Wiggle Test): If the scan tool readings appear normal at times, the issue might be intermittent. Perform a “wiggle test” by gently manipulating the wiring harness and connectors related to the ECT sensor while observing the ECT reading on the scan tool. Look for fluctuations or drops in the reading that might indicate loose connections or wiring problems. Repair any identified issues.

7. Freeze Frame Data Review: If your scan tool supports freeze frame data, review it. This data captures the engine parameters at the moment the P0117 code was set. Check the ECT reading in the freeze frame data. If it shows an extremely high temperature reading at the time of the fault, replacing the ECT sensor is a reasonable next step, especially if initial tests pointed towards a sensor issue.

If, after thoroughly following these diagnostic steps, the P0117 code persists, or if you are uncomfortable performing these procedures, it is recommended to seek professional diagnosis and repair from a qualified automotive technician.

Related DTCs:

• P0115 – ECT Sensor Circuit Malfunction
• P0116 – ECT Sensor Circuit Range/Performance Problem
• P0118 – ECT Sensor Circuit High Input
• P0119 – ECT Sensor Circuit Intermittent
• P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control
• P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

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