Engine

P0115 — Engine Coolant Temperature Sensor Circuit Malfunction

Severity: medium Drive with care, to a shop

What P0115 means

The engine control module sends a reference voltage to the engine coolant temperature (ECT) sensor and reads the voltage change to determine engine temperature. P0115 sets when that signal is out of range, missing or erratic. The module uses this data for fuel delivery, ignition timing and fan control, so a bad signal makes engine management inaccurate.

Symptoms

  • Check Engine light on
  • Hard starting, hot or cold
  • Rough or unstable idle
  • Poor fuel economy
  • Erratic or incorrect temperature readings
  • Cooling fan running too early, too late or constantly

Possible causes

  1. Failed ECT sensor
  2. Damaged wiring or corroded sensor connector
  3. Open circuit or short to ground or power in the signal wire
  4. Poor terminal contact at the control module
  5. Internal control module input fault (rare)

How to diagnose

  1. Read codes and freeze frame data to see the conditions when the fault set
  2. Compare the scan tool ECT reading with actual temperature; a cold engine should match ambient air
  3. Inspect the sensor connector and harness for corrosion, chafing, heat damage or coolant leaks
  4. Measure sensor resistance and compare it with the spec for your engine
  5. Unplug the sensor and check reference voltage and ground at the harness side
  6. Test the signal wire to the module for opens and shorts
  7. Check coolant level and make sure there is no air pocket in the system
  8. If wiring and sensor are good, test the control module

Common fixes

  • Replace the ECT sensor
  • Repair the wire or replace the connector
  • Clean corroded terminals
  • Replace or reprogram the control module (last resort)

P0115 on different brands

Ford · manuals
Ford uses the extended format P0115:16, meaning circuit voltage below threshold; it identifies the condition the monitor detected, not a confirmed failed part.

Questions and answers

Can I drive with a P0115 code?

Briefly and carefully, yes, but the engine runs on default data, so fuel economy and starting suffer. Watch the temperature gauge and get it diagnosed soon.

How do I test the coolant temperature sensor?

Compare the scan tool reading with actual temperature on a cold engine, then measure sensor resistance at different temperatures. It should change smoothly and match the manufacturer's chart.

Why is my fan always running with P0115?

The module can't trust the temperature signal, so it runs the fan to prevent overheating. That is a normal fail-safe; look for the cause in the sensor or its circuit.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.