Auto Repair Q&A



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P0115 and Coolant Temp Sensor Diagnosis

Engine cooling fan

When you receive an error code on your car reading a P0115, your first reaction is likely to quickly replace that coolant sensor. That may fix the problem, but making a guess like that is also likely to cost you the part and time spent replacing it without a fix.

P0115 is the OBD II code for the ECT (engine coolant temp). It indicates that the circuit has malfunctioned or that the engine temperature is regularly going over limits. The result is that the engine is likely to cut back on its capability or shut down altogether in an attempt to save itself from a serious overheat. An engine that shuts off with this code may not restart until the ECT reads an operating temperature or lower.

Check the following before proceeding:

  • Cooling fans are operating to blow air through the radiator.
  • Has the maintenance interval for the anti-freeze/coolant been reached and is it due or overdue for replacement?
  • With the engine cool: is the anti-freeze level at full?

If any of the above are a problem, then your engine may be genuinely overheating and the ECT is likely fine. You can use a "laser" temperature sensor on the engine block and coolant hoses to get a reading on the approximate engine temperature when the ECT shuts the engine down. If these are above normal operating temperature, or very near that, your engine is probably overheating and the ECT is doing its job.

Using a scan tool, you can pull the actual reading from the temperature sensor, which should roughly coincide with the thermometer readings. A long-stemmed thermometer in the radiator can also give you a good reading on engine temp (again, only when it's cool).

Finally, a test of the sensor can be done by putting a fused jumper wire around the sensor's seat (plugs). This will bypass the sensor and give a maximum temperature reading when the keyswitch is turned on (engine off). That's usually about 250 degrees F. If you get nothing, your circuit likely has issues and should be checked.

The problem of an ECT can be as simple as a radiator flush, as easy as an ECT sensor replacement, or as difficult as a circuit check to find the issue and repair the break in the circuit.