Comprehensive diagnostic guide for P0117

Engine Coolant Temperature Sensor Circuit Low Input

What This Code Means

• P0117 is a Powertrain OBD-II diagnostic trouble code indicating the Engine Coolant Temperature (ECT) sensor circuit has a signal input that is lower than expected (low input). In practical terms, the PCM sees the ECT sensor signal at an abnormally low level.
• The Portuguese description aligns with P0117 (ECT sensor circuit low input). This mapping is reflected in the OBD2 code definitions provided in the verified GitHub resource.
• OBD-II codes are standardized; P-codes are part of the Powertrain Codes category (Powertrain Codes). This is described in the general OBD-II references.

Expected symptoms you may encounter (customer-reported and field-observed)

• Dash gauge shows an unusually cold coolant indication or fluctuates oddly, even when engine is at normal operating temperature.
• Hard starting or extended cranking, especially when the engine is warm or at normal operating temperature.
• Rough idle or stalling in some cases, particularly at idle after startup.
• Noticeable fuel trim issues (PCM may lean or enrich over time due to incorrect ECT readings) and potentially reduced fuel economy.
• Check Engine lamp illuminated with P0117 stored or pending.

Common Causes

Note: Percentages below reflect field experience and the need to tailor diagnosis to the specific vehicle. Where the sources don't provide NHTSA-based statistics, percentages reflect ASE-style diagnostic likelihoods.

• Faulty ECT sensor (stuck low or failing): 40-60%

  • The sensor itself can be defective, producing an abnormally low signal independent of actual coolant temperature.
  • Replacing the sensor is a common corrective action when other circuit issues are ruled out.

• Wiring harness issues or poor connections (short to ground, open circuit, chafed wires, corrosion, loose connector): 20-40%

  • Damaged or corroded wiring to the ECT sensor, or a poor connection at the sensor or PCM, can yield a low signal input.
  • Connector damage or contamination can also cause intermittent or persistent low readings.

• Ground or 5V reference problems (PCM-side or harness-side grounding issues, faulty sensor reference): 5-15%

  • A degraded ground or a compromised 5V reference can cause abnormal sensor readings to the PCM, including a low input signal.

• PCM (ECM) input problem (rare): 0-5%

  • Infrequently, the PCM itself can have an input circuit fault. This is much less common but should be considered if all sensor, wiring, and connector checks are negative.

• Coolant-related or environmental factors (indirect impact)

  • If the sensor is physically near excessive heat or coolant leaks into the connector, readings can be affected. This is typically addressed during wiring/connector inspection and sensor replacement.

Important Notes

• Many P0117 scenarios are resolved by addressing wiring/connectors and the ECT sensor itself. If the sensor and harness test good, Attention to the PCM ground/reference and the 5V supply becomes important.
• As always, the specific vehicle and sensor configuration (2-wire vs 3- or 4-wire ECT, different reference wiring) can affect test procedures and expected readings.

Diagnostic Approach

1) Confirm the fault and collect data

• Use a scan tool to confirm P0117 and review freeze frame data. Note the recorded engine temperature vs actual temperature indicators (gauge, temp gauge if present) at the time the code set.
• Check for other related codes (P0115, P0116, etc.) that might indicate related coolant sensor or circuit issues.

2) Visual inspection

• Inspect the ECT sensor and its wiring harness for obvious damage, chafing, abrasion, water intrusion, oil or coolant contamination, or corrosion on the connector pins.
• Disconnect and inspect the connector for bent pins, corrosion, or a loose fit; reseat or clean as needed.

3) Basic circuit checks (safety-first)

• With the ignition OFF, backprobe (or use the service connector as appropriate) to verify presence of the 5V reference at the sensor input pin and a solid ground at the sensor ground pin. After reconnection, start the engine and observe readings on the scan tool.
• Check for a solid ground path from the ECT sensor to the battery negative side. A poor ground can produce erroneous low readings.

4) Sensor data tests (live data)

• Use the scan tool to monitor ECT sensor signal voltage (and, if available, resistance with the engine at known temperatures).
• Cold engine start: note the sensor voltage. A proper cold signal should be within the expected range for a cold condition (actual value varies by vehicle; refer to service data).
• Hot engine test: observe whether the signal voltage increases with rising coolant temperature. If the signal remains near the lower end or does not follow temperature changes, suspect the ECT sensor or its circuit.

5) Sensor resistance test (if sensor removed)

• Remove the ECT sensor and measure its resistance across the terminals at known temperatures (room temperature and a hot reference temperature, such as by immersing in water bath cooled/heated to approximate 20°C and 60-80°C as appropriate for the engine). Compare to the vehicle's service data/specs for the ECT sensor.
• Note: Many OEM sensors are NTC-type (resistance decreases with temperature). If resistance readings do not track expected values, replace the sensor.

6) Circuit integrity testing (wiring and connectors)

• With the engine OFF and the battery disconnected if needed, perform a continuity check between the ECT sensor connector and PCM connector to verify there are no opens in the wiring.
• Check for shorts to ground or short to supply by inspecting the resistance from the signal wire to ground and to battery power as applicable for the vehicle's ECT circuit schematic.
• If the signal wire reads near ground whenever the engine runs or is cranking, suspect a short to ground in the harness or a faulty sensor.

5V reference and PCM checks

• Confirm the 5V reference is present at the PCM side and that the reference is not sagging under load. A weak or unstable reference can yield abnormal sensor readings.
• If the ECT circuit tests OK with the sensor unplugged (default PCM readings) but shows a fault when the sensor is connected, sensor or wiring is implicated rather than PCM.

8) Special case checks

• If all electrical checks pass but the code persists, consider PCM-related fault or software/firmware issue. This is rare but should be considered after exhaustive wiring/sensor testing.

9) Confirmation test

• If the suspect sensor or wiring is replaced, clear the codes and perform a road test or warm-up test to confirm that P0117 does not return and that live data trends match current coolant temperature.

Common Repairs

• Replace the ECT sensor if tests indicate the sensor is faulty or readings do not track temperature properly.
• Repair/replace damaged wiring or connectors to the ECT sensor and re-seat the connector; ensure clean, dry, tight connections.
• Repair grounding issues or improve the 5V reference path as needed.
• If all electrical checks are clean but issues persist, evaluate the PCM as a last resort (less common).

How to document and close the repair

• Record the symptoms observed, the exact DTC, freeze frame data, and the test results (sensor resistance values, voltage readings, continuity checks, and any references to 5V supply).
• Note the exact parts replaced and the post-repair test results (whether P0117 reappeared during road test or after long idle).
• Update customer notes with any identified contributing factors (loose connector, corrosion, or sensor failure) and the expected outcomes.

Safety Considerations

• Always follow vehicle-specific safety procedures when working around cooling systems and electrical circuits.
• Relieve any pressure from the cooling system and ensure the engine is cool before performing sensor work.
• Disconnect the battery when performing wiring repairs or sensor replacement to prevent short circuits.
• Wear eye protection and avoid contact with coolant which is under moderate pressure when components are removed.

Tools and data you'll want

• OBD-II scan tool with live data and freeze frame capability.
• Digital multimeter (DVOM) with backprobe ability or a compatible waveform probe.
• Service manual or data sheet for the specific ECT sensor (to obtain target resistance values and expected voltage ranges at known temperatures).
• Basic hand tools for sensor replacement (sockets, screwdrivers, etc.) and electrical contact cleaner if cleaning connectors.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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