What does it mean?
P0117 ECT (Engine Coolant Temperature) Sensor Circuit Low Voltage: The purpose of this code is as a diagnostic check for a resistance corresponding to a very high engine coolant temperature. The check is enabled whenever the engine computer ECM (Engine Control Module) or PCM (Powertrain Control Module) is powered. If the resistance of the sensor is less (Low Voltage) than a calibration for a period of time the DTC (Diagnostic Trouble Code) will set.
The ECT (Engine coolant temperature) sensor is used to measure the engine temperature and is threaded in the engine coolant jacket and in direct contact with the engine coolant. The ECT is a thermistor, (resistance is inversely proportional to temperature) the resistance of a thermistor decreases as the temperature increases. As the resistance decreases, the voltage input to the engine computer changes with the engine temperature, which provides engine coolant temperature information.
Low coolant temperature has high resistance and high coolant temperature has low resistance (High Voltage/Low Temp [Cold Engine], Low Voltage/High Temp [Warm Engine]), which can be verified with a digital voltmeter. Figure 1 Voltage Divider Network The ECT circuit is a variable ground, which uses a voltage divider network (Figure 1) where the voltage is divided between the sensor input and a sensor ground inside the computer (Black Box).
The computer provides the 5-volt reference signal to the ECT Sensor (Figure 1). When cold, the sensor provides high resistance, which the computer reads as high signal voltage. As the engine warms up, the THERMISTOR sensor resistance becomes lower and the signal voltage drops, so that is the difference.
You do not need to know all of the inner workings of the computer, just that when you check voltage on the yellow wire (Figure 1) and the engine is cold, you should read high voltage at 3-5 volts and at normal operating temperature the signal voltage should be low around 0.47-1.45 volts, see the below table: Vehicle Maker Voltage Cold (volts) Voltage Hot (volts) GM 5.00 0.87 GM Pull-up Resistor 4.95 0.75 FORD 3.52 0.28 Chrysler 3.77 1.45 Chrysler Pull-up Resistor 4.70 1.83 Chrysler ECM Pull-up Resistor 4.20 1.45 Mercedes-Benz 3.50 0.80 Honda 4.70 0.47 Vehicle Maker Resistance Cold (Ohms) Resistance Hot (Ohms) GM 100,000 100 FORD 58,750 1,180 Honda 15,000 250 Nissan 7,000-11,400 180-200 European Bosch 6,500 230 The ECT has a very high resistance when cold and low resistance when hot (see table).
This type of thermistor is called an NTC (negative temperature coefficient), which is opposite to the situation with most other electrical components. So, if the coolant sensor has a poor connection (high resistance) at the wiring connector, the computer will see a higher voltage signal and would supply a richer-than-normal fuel mixture based on the resistance of the coolant sensor and set one of the ECT diagnostic trouble codes (DTC).
Figure 2 Step Up Resistor ECT Circuit Some vehicle manufacturers use a step-up resistor to broaden the range of the ECT sensor (Figure 2). Figure 2 shows an example of a 2-step ECT circuit showing that when the coolant temperature is low, the computer applies a 5-volt reference signal to the ECT sensor through a higher resistor compared to when the temperature is higher.
P0117 will set if the voltage measured by the engine computer (ECM/PCM) is lower due to low resistance (Low Voltage/High Temp [Warm Engine]). The engine computer checks for a resistance corresponding to a very high engine coolant temperature. The check is enabled whenever the ECM is powered. If the resistance of the sensor is less than a calibration for a period of time P0117 will set.