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How to Check & Replace an Engine Coolant Sensor

by Larry Carley copyright 2021 AA1Car.com

The engine coolant temperature (ECT) sensor is a relatively uncomplicated sensor that monitors the internal temperature of the engine. Coolant within the engine cake and cylinder caput(s) absorbs heat from the cylinders when the engine is running. The coolant sensor detects the alter in temperature and signals the Powertrain Control Module (PCM) so it can tell if the engine is cold, warming upward, at normal operating temperature or overheating.

The coolant sensor is extremely of import because the sensor's input to the PCM affects the operating strategy of the entire engine direction organization. That's why the coolant sensor is often chosen the "main" sensor.

Many of the fuel, ignition, emissions and drivetrain functions handled by the PCM are afflicted by the engine's operating temperature. A dissimilar operating strategy is used when the engine is cold than when it is warm. This is done to improve common cold driveability, idle quality and emissions. Consequently, if the coolant sensor fails or is giving the PCM a false reading, it tin can upset a lot of things.

HOW THE COOLANT SENSOR AFFECTS ENGINE OPERATION

Input from the coolant sensor may be used by the PCM for any or all of the following control functions:

* Start upwardly fuel enrichment on fuel injected engines. When the PCM receives a common cold indicate from the coolant sensor, information technology increases injector pulse width (on time) to create a richer fuel mixture. This improves idle quality and prevents hesitation while the cold engine is warming up. As the engine approaches normal operating temperature, the PCM leans out the fuel mixture to reduce emissions and fuel consumption. A faulty coolant sensor that always reads cold may crusade the fuel control arrangement to run rich, pollute and waste fuel. A coolant sensor that ever reads hot may cause cold driveability problems such every bit stalling, hesitation and crude idle.

* Spark accelerate and retard. Spark advance is often limited for emission purposes until the engine reaches normal operating temperature. This as well affects engine functioning and fuel economy.

* Exhaust gas recirculation (EGR) during warm-up. The PCM will not allow the EGR valve to open until the engine has warmed up to ameliorate driveability. If EGR is allowed while the engine is still cold, it may cause a rough idle, stalling and/or hesitation.

* Evaporative emissions command canister purge. Fuel vapors stored in the charcoal canister are not purged until the engine is warm to prevent driveability problems.

* Open/airtight loop feedback command of the air/fuel mixture. The PCM may ignore the oxygen sensor rich/lean feedback signal until the coolant reaches a certain temperature. While the engine is cold, the PCM will remain in "open loop" and keep the fuel mixture rich to better idle quality and cold driveability. If the PCM fails to go into "closed loop" in one case the engine is warm, the fuel mixture will exist likewise rich causing the engine to pollute and waste gas. This status may too lead to spark plug fouling.

* Idle speed during warm-upward. The PCM will ordinarily increase idle speed when a cold engine is first started to prevent stalling and improve idle quality.

* Transmission torque converter clutch lockup during warm-up. The PCM may non lockup up the torque converter until the engine has warmed up to improve cold driveability.

* Operation of the electric cooling fan. The PCM will cycle the cooling fan on and off to regulate engine cooling using input from the coolant sensor. This chore is extremely important to foreclose engine overheating. Note: On some vehicles, a separate coolant sensor or fan switch may be used for the cooling fan circuit only.

TYPES OF COOLANT SENSORS

Nearly coolant sensors are "thermistors" that modify resistance every bit the temperature of the coolant changes. Most are the "NTC" (Negative Temperature Coefficient) type where resistance drops every bit the temperature goes up. With this type of sensor, resistance is high when the engine is cold. Equally the engine warms upwards, the internal resistance of the sensor drops until information technology reaches a minimum value when the engine is at normal operating temperature.

A typical GM coolant sensor, for instance, may have effectually 10,000 ohms resistance at 32 degrees F and drop to under 200 ohms when the engine is hot (200 degrees). A Ford coolant sensor, past comparison, may read 95,000 ohms at 32 degrees and drop to ii,300 ohms at 200 degrees.

Resistance specifications will vary depending on the application, so any sensor that does non read within its specified range should exist replaced.

Coolant sensors have 2 wires (input and render). A 5-volt reference voltage signal is sent from the PCM to the sensor. The amount of resistance in the sensor reduces the voltage bespeak that so returns to the PCM. The PCM then calculates coolant temperature based on the voltage value of the render point. This number can be displayed on a scan tool, and may also be used by the instrument panel cluster or driver information centre to display the temperature reading of the coolant.

On some applications, a "dual range" coolant temperature sensor may exist used. When the coolant reaches a certain temperature, the PCM changes the reference voltage to the sensor and then it can read the coolant temperature with higher accurateness (higher resolution).

On some older vehicles, a unlike type of coolant sensor may exist used. Some of these are essentially an on/off switch that opens or closes at a predetermined temperature. The sensor may exist wired straight to a relay to turn the electric cooling fan on and off, or it may send a signal to a warning light on the instrument panel. These older coolant sensors are typically single wire sensors. On other older applications, a single wire variable resistor temperature sensor that grounds through the threads may be used to send a temperature point to a estimate on the musical instrument panel. These are typically called temperature "senders" rather than sensors.

COOLANT SENSOR LOCATION

The coolant sensor is typically located well-nigh the thermostat housing in the intake manifold. On a few vehicles, the coolant sensor may be located in the cylinder head, or there may be two coolant sensors (one for each cylinder bank in a V6 or V8 engine) or one for the PCM and a second for the cooling fan.

The sensor is positioned so the tip will be in direct contact with the coolant. This is essential to produce a reliable signal. If the coolant level is depression, information technology may prevent the coolant sensor from reading accurately.

COOLANT SENSOR SYMPTOMS

Because of the coolant sensor'due south central function in triggering and so many engine functions, a faulty sensor (or sensor circuit) volition often crusade cold driveability and emission problems. A bad coolant sensor can as well crusade a noticeable increment in fuel consumption, and information technology may cause a vehicle to neglect an emissions test if it prevents the engine direction system from going into closed loop.

Keep in mind that many coolant sensor problems are more oft due to wiring faults and loose or corroded connectors than failure of the sensor itself.

The coolant sensor's affect on the engine direction system, cold driveability, emissions and fuel economic system can also be influenced by the thermostat. If the thermostat is stuck open up, the engine will be tiresome to warm upwards and the coolant sensor will read low. Or, if someone installed the wrong thermostat for the awarding or removed the thermostat altogether, it will prevent the engine from reaching normal operating temperature and cause the coolant sensor to read depression.

A faulty coolant sensor may also crusade the engine to overheat if it fails to energize the cooling fan relay when the engine gets hot.

A faulty coolant sensor may likewise cause inaccurate coolant temperature gauge readings on the instrument panel.

COOLANT SENSOR DIAGNOSTIC FAULT CODES

On 1996 and newer vehicles with OBD II onboard diagnostic systems, a faulty coolant sensor may prevent some of the system monitors from running. This volition preclude the vehicle from passing an OBD Two emissions test considering the test can't exist washed unless all the required system monitors take run and passed.

The OBD Two system should catch the error, turn on the Check Engine Calorie-free or Malfunction Indicator Lamp (MIL), and set one of the following diagnostic trouble codes:

P0115....Engine Coolant Temperature Circuit
P0116....Engine Coolant Temperature Circuit Range/Performance
P0117....Engine Coolant Temperature Circuit Depression Input
P0118....Engine Coolant Temperature Circuit High Input
P0119....Engine Coolant Temperature Circuit Intermittent

On older pre-OBD II vehicles, the Check Engine light may come up on if the coolant sensor is shorted, open or is reading out of range. GM coolant sensor codes include codes 14 & 15, Ford codes are 21, 51 & 81, and Chrysler codes are 17 & 22.

COOLANT SENSOR DIAGNOSIS

1 style to quick bank check the accuracy of your engine's coolant sensor is to claw up a scan tool to the OBD diagnostic connector. Pull up the sensor data menu and compare the coolant sensor temperature reading and the air temperature sensor reading when the engine is Cold. Both sensors should read within a degree or two of each other. If the readings are significantly dissimilar, one of the sensors is bad.

With the scan tool still plugged in, start the engine and picket the sensor readings as the engine warms up. The ambient air temperature reading should remain about the same but the coolant temperature sensor should prove a gradual rise in coolant temperature. No change in the coolant sensor reading would tell you the sensor is bad.

On 1996 and newer vehicles with OBD II, your scan tool should be able to brandish the coolant sensor's output in degrees Centigrade (C) or Fahrenheit (F). The coolant sensor should read low (or ambience temperature) when the engine is cold, and loftier (effectually 200 degrees) when the engine is hot. No change in the reading or a reading that obviously does not match engine temperature would indicate a faulty sensor or a wiring problem.

The internal resistance of a coolant sensor can also be checked with an ohmmeter or DVOM (digital volt ohm meter) and compared to specifications. If the sensor is open, shorted or reads out of range, it must be replaced.

If the resistance of a coolant sensor is inside specifications and changes as engine temperature changes, but the engine is not going into airtight loop, the fault is in the wiring or PCM. Further diagnosis volition be needed to isolate the problem earlier whatsoever parts are replaced.

1 trick here is to employ a sensor simulator tool to feed a simulated temperature reading through the sensor's wiring harness to the PCM. If the wiring continuity is practiced simply the PCM fails to get into closed loop when you lot send it a "hot coolant" point, the trouble is in the PCM.

A visual inspection of the coolant sensor should too exist made to check for issues such equally severe corrosion effectually the terminal, broken or loose wires, a fissure in the sensor, or coolant leaks around the base of the sensor.

COOLANT SENSOR VOLTAGE CHECKS

You tin can also use a voltmeter or digital storage oscilloscope (DSO) to bank check the sensor's output. Specs vary, only generally a cold coolant sensor will read somewhere around 3 volts. Equally the engine warms up and reaches operating temperature, the voltage drop should gradually decrease down to about ane.2 to 0.5 volts. If y'all're using a scope to brandish the voltage signal, you should get a trace that gradually slopes from iii volts down to 1.2 to 0.five volts in 3 to five minutes (or all the same long information technology unremarkably takes the engine to reach normal operating temperature).

If the voltage drop beyond the coolant sensor reads at or near 5 volts, it means the sensor is open up or information technology has lost its footing connexion. If the voltage is close to aught, the sensor is shorted or it has lost its reference voltage.

When working on older 1980s vintage Chrysler products, watch out for a sudden voltage increment as the engine warms upward. This is normal and is produced past a thousand ohm resistor that switches into the coolant sensor excursion when the sensor'due south voltage drops to most one.25 volts. This causes the voltage to jump dorsum up to about 3.7 volts, where it again continues to drop until it reaches a fully warmed up value of about 2.0 volts.

Sometimes a coolant sensor will all of a sudden go open up or short when it reaches a sure temperature. If your voltmeter has a "minimum/maximum" part, you lot tin can take hold of sudden voltage fluctuations while the sensor is warming upwardly. If yous are viewing the voltage pattern on a scope, a brusque volition appear as a sudden drop or dip in the trace to zero volts. An open up would make the trace jump up to the VRef voltage line (5 volts).

If the coolant sensor reads ordinarily when cold (loftier resistance and three or more volts), but never seems to reach normal temperature it could be telling the truth! An open up thermostat or the incorrect thermostat may be preventing the coolant from reaching its normal operating temperature.

COOLANT SENSOR REPLACEMENT

Most coolant sensors are non replaced unless they have failed. A coolant sensor that is shorted, open up or reading out of range plain tin can't provide a reliable temperature bespeak and must be replaced for the engine management system to function properly. But many experts also recommend installing a new coolant sensor if you lot are replacing or rebuilding an engine. Why? Because coolant sensors can deteriorate with historic period and may not read as accurately as they did when they were new. Installing a new sensor tin eliminate a lot of potential problems down the road.

It is likewise a proficient thought to supervene upon the coolant sensor and thermostat if the engine has experienced a case of severe overheating. Abnormally high engine temperatures can damage these components and may cause them to misbehave or fail prematurely.

Replacing a coolant sensor requires draining some of the coolant from the cooling arrangement. You do non take to drain the unabridged radiator. Just open the drain valve and let out enough coolant so the coolant level in the engine is below the sensor.

This would exist a good time to check the status of the coolant, and to replace information technology if the coolant is more than three years old or 50,000 miles (conventional "green" coolant) or ten years or 100,000-plus miles (long life coolant). A coolant modify and a flush would too exist a expert idea if the coolant shows whatever signs of contamination.

The threads on the coolant sensor may exist pre-coated with sealer to forestall coolant leaks. Tighten the sensor carefully to prevent damage.

Once the new sensor has been installed, you lot can refill the cooling system. Make sure all the air is out of the cooling organization. Air trapped under the thermostat may cause the engine to overheat or the coolant sensor to not read correctly.

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