P0115 – Engine coolant temperature (ECT) sensor circuit malfunction


By Reinier (Contact Me)
Last Updated 2016-07-09
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P0115 Engine coolant temperature -(ECT) sensor circuit malfunction
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What Does Code P0115 Mean?

OBD II fault code P0115 is defined as “Engine Coolant Temperature Circuit Malfunction”, and is set when the PCM (Powertrain Control Module) detects a circuit voltage value in the engine coolant temperature sensor that falls outside of a predetermined range.

On most applications, a code will be stored and a warning light illuminated when the PCM detects a voltage/resistance value that deviates to either side from design parameters by about 10%, and/or for a specified time. Note that both the degree of deviation from specified electrical values and the time required before a code sets can vary between applications. Consult the repair manual for the application being worked on for the exact code setting parameters.

Temperature sensors on modern engines are of the variable resistance type that converts temperature into signal voltages. In general, the cooler the engine coolant, the higher the resistance but as the engine coolant temperature increases, resistance drops, thereby increasing the signal voltage to the PCM.

Note that it is rare for an engine to have only one engine coolant temperature sensor. Most applications use one sensor to control the heat gauge, and another that communicates with the PCM to control the electric radiator fan, and/or cold start valves on some older applications.  Some diesel engines may have a dedicated coolant temperature sensor to control the working of the glow plugs.

The image below shows a typical engine coolant temperature sensor location. Note that actual designs and locations differ widely between applications.

Coolant sensor

What are the common causes of code P0115 ?

Possible causes of code P0115 could include the following. Note however that some “causes” may be the effects of unrelated cooling system problems, such as severe engine overheating that can damage sensors and wiring.

  • Burnt, damaged, shorted, or corroded wiring and connectors.
  • Defective coolant temperature sensors.
  • Low engine coolant levels. Note that other warning lights and codes are likely to be present when coolant levels are low.
  • Poor coolant circulation.
  • Failed, or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P0115 ?

In most cases of a simple failure of engine coolant temperature sensors’ control circuits, the only symptom that is likely to be present is a stored trouble code and an illuminated warning light.

How do you troubleshoot code P0115 ?

NOTE#1: Secondary effects of not resolving code P0115 in good time could lead to engine failure due to fatal overheating. Engine overheating can precipitate the failure of the coolant temperature sensor’s control circuit due to the high overall temperature of the engine; and since it is sometimes impossible to differentiate between the causes and effects of engine overheating, diagnosing code P0115 should include a thorough inspection of the entire engine cooling system as a matter of course.

NOTE#2: Before starting to diagnose code P0115, consult the manual on the number, location, and function of each of the various engine temperature sensors if the engine has more than one sensor. Code P0115 does not indicate which coolant temperature sensor’s control circuit is defective, which makes it easy for the novice DIY mechanic to investigate the wrong circuit, or to replace the wrong sensor with possible dire consequences.

NOTE#3: To avoid confusion and a possible wrong diagnosis on applications with more than one coolant temperature sensors, it is important to determine the following-

  • Did the engine overheat?
  • Did the temperature gauge work correctly before the code appeared?
  • If the engine did not overheat, is the temperature gauge still registering the coolant temperature? Let the engine idle for a few minutes to verify if the temperature gauge is working or not.
  • If the temperature gauge does register the engine coolant temperature, it is obvious that the coolant temperature sensor (or its control circuit) that communicates with the PCM is defective. If this is the case, the operation of the electrical radiator fan will almost certainly be affected.

Use the above questions and checks to isolate the defective sensor and/or its control circuit. Note that if the engine did overheat, there is little point in continuing the diagnostic/repair process. In these cases, the engine must be replaced, or suitable repairs made to get the engine running again.

Step 1

Assuming for the purposes of this guide that the engine did not overheat, record all codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

WARNING: Even if the engine did not overheat, it is important at this point to inspect the entire cooling system for leaks or other visible problems, such as broken drive belts, excessive free play in the water pump, or ruptured radiator hoses. Low coolant levels brought about by leaks in the system can affect the working of coolant temperature sensors, which might be interpreted by the PCM as defects in the sensors’ control system.

Bear in mind that circulation issues caused by a defective thermostat can have the same effects as low coolant levels in the sense that hot coolant is not brought into contact with the sensor, which the PCM might also interpret as a control circuit defect.  It is also important to test the coolant for the presence of hydrocarbons, which indicates an internal coolant leak. Depending on the leak path, coolant circulation could be interrupted, which the PCM might also interpret as a defect in a temperature sensor’s control circuit.

Step 2

If it is certain the engine coolant system is fully functional, and there are no fluid leaks, use the guidelines in NOTE #3 to identify the defective sensor and its control circuit. Consult the manual on the location, function, colour-coding, and routing of each wire in the circuit.

Look for burnt, shorted, damaged, and/or corroded wiring and connectors. Note that exposure to high engine temperatures over long periods can make the insulation on wiring brittle, which can cause intermittent short circuits. To prevent the possibility of this happening in future, replace all wiring on which the insulation is hard, brittle, or flaking off. When replacing wiring, be sure to use wire of the same gauge and color to avoid confusion and mistakes later on.

Step 3

If no visible problems on the wiring are found, perform resistance, reference voltage, ground, and continuity checks on all associated wiring. Although the reference voltage for this circuit is usually 5 volts, consult the manual for the exact value. Be sure to disconnect the sensor from the PCM (if applicable) before starting continuity checks to prevent damage to the controller. The affected sensor forms part of the control circuit, and as such its internal resistance must be tested for as well. This test requires the manufacturer’s temperature to resistance chart.

Replace the sensor if obtained resistance readings do not comply with the manufacturer’s specifications. Note that on some applications, the various radiator fan speeds are controlled with resistors. The PCM switches the fan’s voltage to one or more resistors, based on the engine coolant temperature. Be sure to consult the manual on the location, and resistance values of these resistors, and replace all resistors that do not conform to the manufacturer’s specifications.

NOTE: Resist the temptation to pour boiling water over the sensor body to obtain a resistance reading. Engine coolant does not reach boiling point, and using boiling water from a kettle will thus give a misleading reading. Always follow the testing procedure outlined in the repair manual for the most accurate results.

Step 4

If repairs or parts replacements involve the coolant temperature sensor that controls the radiator fan are complete, try to activate the radiator fan with the scanner if the scanner has that functionality.  With many scanners it is possible to activate the radiator fan to run at all its speeds, but pay close attention to the displayed fan RPM’s, and compare these readings to the values stated in the manual.

Also compare the displayed current draw to the values stated in the manual. The actual current draw should match the stated value very closely, if not exactly. Significant deviations will not necessarily set a code on all applications, which means that deviations between actual and recommended current draws must be investigated and resolved.

Step 5

If the radiator fan and/or temperature gauge cannot be activated manually, clear all codes and retest the system after repairs had been made to ensure/verify that all electrical readings fall within the manufacturer’s specifications.

WARNING: Do NOT operate the vehicle before verifying that all electrical values on all engine coolant temperature sensors conform to the manufacturer’s specifications as stated in the repair manual.

Step 6

Only when it has been established that there are no electrical issues in the control circuits of all coolant sensors and that all codes are cleared, check the engine coolant, close the radiator, and let the engine run at idle with the scanner connected.

Narrow the scanner monitoring function down so that it monitors only the affected/repaired sensor and its control circuit. There should be a gradual change in the signal voltage as the engine coolant heats up. As a double check, confirm this rise in temperature by monitoring the temperature gauge in the dashboard.

Compare the temperature at which the radiator fan comes into operation with the displayed temperature when the fan starts. These two values should be in very close agreement; it they are not, there is still a problem in either a sensor or a sensor control circuit that must be investigated and resolved.

Repeat all resistance, reference voltage, ground, and continuity checks on all associated wiring to trace the problem. If needs be, replace all associated wiring with new wire and connectors, and ensure that all connections and terminations are secure. Repeat Step 6 until the radiator fan starts at the correct coolant temperature, runs at the correct speed for the correct period of time, and at an acceptable current draw.

Step 7

If the radiator fan is in perfect working order and it is certain that the coolant is circulating freely, the only two things that can influence its operation is either its input current, or an intermittent fault in a coolant temperature sensor’s control circuit.

If despite all repair attempts and a correct fan input voltage the radiator fan still does not perform to specifications, it is likely that the problem is intermittent in nature. Intermittent faults can sometimes be extremely difficult to find and resolve, and in some cases it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Step 8

Only road test the vehicle when it is absolutely certain that all electrical values in the control circuits of all fitted coolant temperature sensors fall within specifications and that the radiator fan and/or temperature gauge both work as they should.

Test drive the vehicle with a scanner connected to monitor the real-world performance of all coolant sensors, as well as the radiator fan. As stated before, it is sometimes very difficult, if not impossible to differentiate between cause and effect with this type of code; this means that it may sometimes be necessary to repeat several steps in the diagnostic/repair procedure several times before a repair can be considered to be definitive and reliable.

Codes Related to P0115

  • P0117 – Relates to “Engine Coolant Temperature Circuit Low Input.”
  • P0118 – Relates to “Engine Coolant Temperature Circuit High Input.”
  • P0119 – Relates to “Engine Coolant Temperature Circuit Intermittent.”