Engine Coolant Temperature (ECT) Circuit Malfunction


A P0115 Indicates that the PCM has detected an engine coolant temperature circuit voltage reading that is not within an acceptable limit, within a set period of time.

Code Set Parameters

Engine temperature sensor system circuit voltage that exceeds variations of greater than 10-percent from the manufacturer’s reference voltage/resistance will cause a trouble code to be stored and a malfunction lamp illuminated.


A trouble code will be stored and a malfunction indicator will be illuminated.

Common Causes

Possible causes of this code include a loose ECT sensor electrical connector, faulty ECT circuit wiring, faulty ECT sensor, low engine coolant, or a faulty PCM. PCM failures are rare.

Common Misdiagnosis

Snap judgments often condemn the ECT sensor when the problem proves to be burnt, corroded, or broken electrical wiring. Most domestic models also use separate temperature sensors for the gauge and for the PCM inputs, replacing the wrong sensor can also be a potential hazard for the novice.


  • The engine is equipped with a temperature sensor (usually a two or three-wire variable resistance sensor) which provides the PCM with data
  • The sensor has a voltage wire (typically it is a 5-volt reference signal) and a ground wire
  • The PCM uses the reference voltage wire to monitor engine temperature
  • As engine temperature increases, sensor resistance decreases and reference voltage increases
  • When the engine is cool, sensor resistance is high, driving reference voltage to the PCM down
  • The PCM receives input reference voltage readings as variations in engine temperature and reacts accordingly
  • If engine temperature is excessive, a code is stored and a service engine soon lamp is illuminated. Any vehicle, in which the engine has excessively overheated, should undergo a thorough diagnosis of the engine cooling system before any repairs are performed
  • In most cases this diagnosis should include a preemptive chemical block test
  • This test uses a special application device to capture steam from the engine cooling system where a chemical agent reacts to any hydrocarbons found therein
  • By testing for hydrocarbons in the engine cooling system, the technician can make sure that the replacement of such components as the thermostat, water pump, coolant hoses, heater core, or radiator will effectively repair the overheating condition by ruling out block and cylinder head (or cylinder head gasket) damage
  • After the possibility of a faulty engine block or cylinder head has been eliminated, correcting the problem can be a simple as using a cooling system pressure tester to locate a coolant leak, and then repairing it
  • If no coolant leaks are found, suspect the thermostat, cooling fan, or radiator (circulation). If the engine appears to be in good working order and doesn’t appear to be overheating, suspect a faulty engine temperature sensor
  • A scanner or code reader, a digital volt ohmmeter, and access to a manufacturer’s wiring schematic will be necessary to successfully diagnose this code
  • Begin your diagnosis with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Continue by unplugging the electrical connector from the engine coolant temperature sensor and testing for reference voltage
  • This is usually 5-volts but consult your manufacturer’s service manual to be sure
  • If there is a 5-volt signal present with the ignition turned to the run position, then check the sensor ground wire
  • If both the reference signal and the ground signal are present, test the sensor resistance using the manufacturer’s temperature to resistance chart
  • Compare your findings to the manufacturer’s specifications and replace the sensor if it fails to comply. If the sensor and voltage at the sensor are within acceptable specifications, disconnect the PCM electrical connector and test circuit continuity and resistance between the sensor and the PCM
  • Repair or replace system circuitry and components as required and retest the system. Should all system circuitry and sensors check out, suspect a faulty PCM but keep in mind that PCM failure is rare and PCM replacement will require reprogramming.