Engine Coolant Temperature Sensor Circuit Low Input
The P0117 code indicates that the Engine Control Module (ECM) has detected a voltage signal from the Engine Coolant Temperature (ECT) sensor that is lower than the expected range (typically below 0.2 volts). This usually means the sensor is reading an extremely cold temperature (e.g., -40°F) or there is a short to ground in the sensor circuit. The ECT sensor is a thermistor that changes resistance with temperature; a low voltage suggests low resistance, often due to a short circuit or a failed sensor.
01. Most Common Causes
- Faulty Engine Coolant Temperature (ECT) sensor
- Short to ground in the ECT sensor signal circuit
- Open or shorted wiring in the ECT sensor harness
- Poor electrical connection at the ECT sensor connector
- Faulty Engine Control Module (ECM) (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Hard starting, especially in cold weather
- Poor fuel economy and increased emissions
- Engine may run rich or lean, causing rough idle or hesitation
- Cooling fan may run continuously or not at all
03. Step-by-Step Diagnosis
Step 1: Connect a scan tool and verify the P0117 code. Check freeze frame data for coolant temperature reading (should be near ambient if engine is cold).
Step 2: Visually inspect the ECT sensor connector and wiring for damage, corrosion, or loose connections. Repair as needed.
Step 3: Disconnect the ECT sensor and measure resistance across its terminals. Compare to specifications (e.g., 2-3 kΩ at 68°F). If out of range, replace sensor.
Step 4: With the sensor disconnected, check for voltage at the harness connector (key on, engine off). Should be around 5V reference. If 0V, check for open/short in wiring or ECM issue.
Step 5: If voltage is present, jumper the signal and ground terminals and check if scan tool reads a high temperature (e.g., 250°F). If not, there may be a wiring issue. If it does, replace the sensor.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. The ECM may use a default temperature value, causing poor engine performance, increased emissions, and potential overheating or overcooling. Driving short distances to a repair shop is acceptable, but prolonged driving could damage the engine.
How much does it cost to fix?▼
Typically between $50 and $150. The ECT sensor itself costs $20-$50, and labor is about $30-$100 depending on vehicle accessibility. Diagnosis may add $50-$100 if performed by a shop.