|Trouble Code||Fault Location||Probable Cause|
|P0118||Engine coolant temperature (ECT) sensor -high input||Coolant thermostat, wiring open circuit/short to positive, earth wire defective, ECT sensor|
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What Does Code P0118 Mean?
P0118 Engine Coolant Temperature Sensor Circuit Hi Voltage: diagnostic checks for a resistance corresponding to a very low engine coolant temperature. Engine control module (ECM) or PCM (powertrain control module) supplies 5 V to the ECT sensor signal circuit and a ground for the low reference circuit. Diagnostic checks for an open, short to ground or an intermittent circuit condition between the ECM and ECT sensor. The engine must run for a calibration period of time before the engine coolant temperature sensor high check is executed. This allows the engine time to heat the coolant up beyond the bottom of the sensor range in cold ambient conditions. If enable conditions are met and the resistance of the sensor is greater than a calibration for a period of time the test will fail and the code sets.
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 Pull-up Resistor||4.95||0.75|
|Chrysler Pull-up Resistor||4.70||1.83|
|Chrysler ECM Pull-up Resistor||4.20||1.45|
|Vehicle Maker||Resistance Cold (Ohms)||Resistance Hot (Ohms)|
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.
P0118 will set if the voltage measured by the engine computer (ECM/PCM) is higher due to high resistance (Low Voltage/High Temp [Warm Engine]) like a disconnected ECT sensor. The engine computer checks for a resistance corresponding to a very low engine coolant temperature. The check is enabled whenever the ECM is powered. If the resistance of the sensor is greater than a calibration for a period of time P0118 will set.
What are the common causes of code P0118 ?
- Bad connection at the sensor
- Short in the voltage feed between the sensor and ECM/PCM
- Defective ECM/PCM (not likely)
- Defective ECT sensor (shorted internally)
What Happens When DTC P0118 Sets
- Cooling fans will be commanded ON
- Engine Coolant Temperature Gauge is inoperative
- AC compressor will be commanded OFF
Conditions for Setting DTC P0118
- ECM/PCM detects that the ECT is colder than −38°F (−39°C) for more than five seconds.
- Ignition is ON or engine is running for more than 10 seconds.
- Engine run time is less than 10 seconds when IAT Sensor 1 is warmer than –7°C (19°F).
- DTC P0118 runs continuously within the enabling conditions.
What are the symptoms of code P0118 ?
- MIL (Malfunction Indicator Lamp) [Check-Engine Light) illumination
- Poor fuel economy
- No start condition
- Vehicle starts, but runs very poorly, running very rough and misfiring
How do you troubleshoot code P0118 ?
Tools Needed: Scan Tool (rented or purchased, about $250), If you have a laptop PC, a good choice is www.autoenginuity.com for $247, DMM (Digital Multimeter), and access to an online service information web site like www.alldata.com
Scan Tool Testing
- If not already done, go to an online service information provider like ALLDATA or Mitchell On-Demand or equivalent and download the P0118 diagnostic procedure and diagnostic circuit check. Also do a TSB (Technical Service Bulletin) search for issues related to DTC P0118.
- Turn the ignition ON and connect a scan tool or laptop computer with the appropriate interface and software using the instructions provided with the scan tool to the vehicle DLC (data link connector). If the scan tool does not work, check, use the diagnostic circuit check you downloaded in step 1.
- You will have the MIL light on, so verify the that DTC P0118 is stored or active
- Compare the temperature of the engine coolant as displayed on your scan tool with the actual temperature of the engine and record the scan tool engine temperature.
- Measure the actual temperature of the coolant using an infrared pyrometer or contact-type temperature probe. Animation: Verify Engine Operating Temperature (View) (Download)
- Maximum difference between the two readings should be 10°F (5°C).
- If actual engine temperature varies by more than 10°F (5°C) from the scan tool temperature, check the ECT sensor wiring and connector for damage or corrosion.
- To check that the wiring and see if the computer is working okay, with scan tool still connected, look at the ECT temperature and unplug the connector from the ECT sensor. The temperature on the scan tool should read about −40° F. With the connector still removed from the ECT, use a fused jumper lead and connect the 2 terminals of the connector together, the scan tool should display approximately 285°F (140°C).
- If the above test works, the connector and wiring are most likely okay, check the ECT sensor for resistance and compare to the actual engine temperature chart in Step 7 below.
- If the wiring does not work as indicated in Step 4 third bullet, Check for an open in the yellow signal wire or a disconnected or corroded ECT sensor connector, see Figure 1 or the downloaded wiring diagram.
- Testing Engine Coolant Temperature Sensor
- Check that radiator reservoir bottle is full and radiator itself is filled to top.
NOTE: Be sure that the radiator is cool before removing the radiator cap to avoid being scalded by hot coolant. Normal operating temperature varies with vehicle make and model. Some vehicles are equipped with a thermostat with an opening temperature of 180°F (82°C), whereas other vehicles use a thermostat that is 195°F (90°C) or higher. Before replacing the ECT sensor, be sure that the engine is operating at the temperature specified by the manufacturer. Most manufacturers recommend checking the ECT sensor after the cooling fan has cycled twice, indicating a fully warmed engine.
- ECT sensor must be submerged in coolant to be able to indicate the proper coolant temperature.
- Proper pressure maintained by the radiator cap. If the radiator cap is defective and cannot allow the cooling system to become pressurized, air pockets could develop. These air pockets could cause the engine to operate at a hotter than-normal temperature and prevent proper temperature
- Check the antifreeze–water mixture. Most OEMs (Original Equipment Manufacturers) recommend a 50/50 mixture of antifreeze and water
- Check for proper operation of the cooling fan. If the cooling fan does not operate correctly, the engine may overheat.
- Go to an online service information provider like ALLDATA or Mitchell On-Demand or equivalent and download the Temperature Versus Resistance – Engine Coolant Temperature Sensor table for ECT for the vehicle you are working on.
- With the Ignition OFF, disconnect the harness connector at Engine Coolant Temperature Sensor.
- Test the ECT sensor while running the engine to vary the sensor temperature while monitoring the sensor resistance with DMM ohmmeter function: Animation: Test Engine Coolant Temperature ECT Sensor (View) (Download). Compare the readings with the downloaded table. The resistance values should be in range of the table values. You can also check the resistance of the sensor with the DMM ohmmeter function as also shown in the animation. Check the downloaded service information for the right specification.
- If not within the specified range replace the ECT sensor.
- If within the specified range, test for infinite resistance between each terminal and the sensor housing.
- If less than infinite resistance, replace the ECT sensor.
- If infinite resistance is okay, check the computer and consult a professional technician or service shop.
Optional Detailed ECT Circuit Testing if All Else Fails
- Go to an online service information provider like ALLDATA or Mitchell On-Demand or equivalent and download the wiring diagram for ECT for the vehicle you are working on.
- Study the wiring diagram and using a highlighter trace the circuit from the ECM (Engine Control Module) or PCM (Powertrain Control Module) to the ECT.
- With the ignition OFF and all vehicle systems OFF, disconnect the harness ECT connector.
- Test for < 5 ohms between the low reference terminal and ground.
- If 5 ohms or greater, turn the ignition OFF, disconnect the harness connector at the ECM or PCM.
- Test for less than 2 ohms in the low reference circuit end to end, review wiring diagram.
- If 2 ohms or more, repair the open/high resistance in the circuit.
- If less than 2 ohms, the ECM could be defective.
- If less than 5 ohms with the ignition ON.
- Verify the scan tool ECT Sensor parameter is colder than −39°C (−38°F).
- If warmer than −38° F (−39° C) with ignition OFF, disconnect the harness connector at the ECM/PCM.
- Test for infinite resistance between the signal circuit terminal and ground.
- If less than infinite resistance, repair the short to ground on the circuit.
- If infinite resistance, the ECM may be defective.
- If colder than −38° F (−39° C)
- Install a 3 A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal.
- Verify scan tool ECT sensor parameter is warmer than 300° F (149° C).
- If 300° F (149° C) or colder
- Ignition OFF, disconnect the harness connector at the ECM/PCM
- Turn the ignition ON, using a DMM (Digital Multimeter) Test for less than 1 volt between the signal circuit and ground.
- If 1 volt or greater, repair the short to voltage on the circuit.
- If less than 1 volt, turn the ignition OFF and test for less than 2 ohms in the signal circuit end to end.
- If 2 ohms or greater, repair the open/high resistance in the circuit.
- If less than 2 ohms, the ECM is defective.
- If warmer than 149° C (300° F): Test or replace the Engine Coolant Temperature Sensor.
DTC P0118 Diagnostic Tips
- Coolant temperature gauge will display full cold when an ECT DTC set’s.
- After starting the engine, the ECT sensor temperature should rise steadily, then stabilize after the thermostat opens.
DTC P0118 Scan Tool Data Parameters
Short to Ground
Short to Voltage
Operating Conditions: Engine running at various operating conditions
Parameter Normal Range: Varies with coolant temperature
Codes Related to P0118
Short to Ground
Short to Voltage
*ECM/PCM or sensor damage may occur if the circuit is shorted to B+.
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