What Does P0115 Mean?
The ECM has detected a general malfunction in the Engine Coolant Temperature (ECT) sensor circuit 1. The ECT sensor is critical for fuel injection, ignition timing, idle speed, transmission shift strategy, and cooling fan operation. Without an accurate coolant temperature signal, the ECM uses a default value. The driver may notice extended warm-up periods, incorrect temperature gauge reading, poor fuel economy, the cooling fan running at the wrong times, or the engine running too rich during warm-up.
Common Causes
35%
Failed ECT sensor (open or shorted internal thermistor)
25%
Corroded or disconnected ECT sensor connector
20%
Damaged wiring in the ECT sensor circuit (open or shorted)
10%
Coolant contamination or air pocket around the ECT sensor giving inaccurate readings
10%
ECM internal fault on ECT input circuit
Diagnostic Steps
1
Check live ECT data on a cold engine — it should read within a few degrees of ambient temperature. Compare to the IAT sensor reading for reasonability.
2
Start the engine and monitor the ECT reading during warm-up — it should rise steadily from ambient to approximately 195-220°F (90-105°C) within 5-10 minutes. A reading stuck at -40°F, 280°F, or not changing at all indicates a circuit fault.
3
Disconnect the ECT sensor and measure its resistance. At 68°F (20°C), expect approximately 2.0-3.5 kOhms; at 212°F (100°C), expect approximately 180-300 ohms. Compare to manufacturer specs.
4
Inspect the ECT sensor connector for coolant residue (indicating a leaking sensor), corrosion, or damaged terminals.
5
Check wiring continuity and insulation from the ECT sensor connector to the ECM on both the signal and ground return wires.
Estimated Repair Cost
$20 - $175
Parts + labor, varies by vehicle and location
The ECM has detected a general malfunction in the Engine Coolant Temperature (ECT) sensor circuit 1. The ECT sensor is critical for fuel injection, ignition timing, idle speed, transmission shift strategy, and cooling fan operation. Without an accurate coolant temperature signal, the ECM uses a defa...
The most common cause of P0115 (Engine Coolant Temperature Sensor Circuit 1 Malfunction) is: Failed ECT sensor (open or shorted internal thermistor)
Typical repair costs for P0115 range from $20 to $175, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Driveable but monitor your actual engine temperature carefully using an infrared thermometer or by feeling the upper radiator hose. Without a functioning ECT, the cooling fans may not engage at the correct temperature, potentially leading to overheating. If you notice any signs of overheating (steam, hot smell), stop immediately.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0115 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Cooling / Fuel Management
Difficulty
Type
Generic (SAE)
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