P0115

Engine Coolant Temperature Sensor Circuit 1 Malfunction

P0115 is a generic OBD-II powertrain diagnostic trouble code: Engine Coolant Temperature Sensor Circuit 1 Malfunction. It is logged by the engine control unit when the coolant monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0115
Group
Powertrain
System
Coolant
Severity
Warning (MIL on)
Need P0115 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P0115 means

P0115 is stored when the ECM/PCM detects a voltage from the Engine Coolant Temperature (ECT) sensor circuit that falls outside its expected range — typically below 0.14 V or above 4.91 V — indicating either an open circuit or a hard short. The ECT sensor is a negative-temperature-coefficient (NTC) thermistor: its resistance is high when the coolant is cold and drops as temperature rises, producing a high voltage signal when cold and a low voltage signal when hot. A signal outside these bounds means the PCM cannot determine actual coolant temperature.

Coolant temperature data is used by the ECM to control fuel enrichment during cold starts, idle speed, ignition timing, exhaust gas recirculation, cooling-fan operation, and emissions systems. When the signal is lost or out of range, the ECM substitutes a default value — often a fixed mid-range temperature — which can result in either a lean cold-start condition or an over-enriched warm-running condition depending on which direction the circuit has failed. Long warm-up times, rough cold idle, and poor fuel economy are common driver complaints.

Most failures trace to a corroded or damaged connector, a broken wire in the harness near the sensor, or a sensor that has failed open internally. Low coolant level can also contribute if the sensor tip is no longer immersed. The fix is almost always inexpensive: repair or replace the harness connector, or swap the sensor itself.

Common causes

Most-frequently reported root causes when P0115 is logged.

  • 1
    Defective ECT sensor with an open or short-circuit internal failure.
  • 2
    Corroded, bent, or improperly seated connector pins at the sensor plug.
  • 3
    Broken, chafed, or open-circuit wire in the ECT sensor harness.
  • 4
    Short circuit in the sensor signal or ground wire to chassis ground or to battery voltage.
  • 5
    Low engine coolant level leaving the sensor tip partially or fully exposed to air.
  • 6
    Air pocket trapped around the sensor in the coolant passage, causing erratic temperature readings.
  • 7
    Faulty PCM/ECM reference or ground circuit (uncommon; rule out all external causes first).

Symptoms drivers notice

Check engine light (MIL) illuminated.
Extended warm-up time or poor cold-start performance with rough idle.
Poor fuel economy caused by the ECM using an incorrect temperature substitute value.
Engine may run slightly rich or lean depending on the direction of the circuit failure.
Cooling fans may run continuously or not at all if the PCM loses temperature reference.
In severe cases, risk of engine overheating if the fan control logic depends on ECT data.

How to diagnose P0115

A typical diagnostic flow when this code is present.

  1. 1
    Connect an OBD-II scanner and compare the live ECT reading against actual coolant temperature (infrared thermometer on the thermostat housing); more than 10 °F difference is suspect.
  2. 2
    Inspect the ECT sensor connector and harness for corrosion, broken wires, or poor pin contact; clean or repair as needed.
  3. 3
    With the connector unplugged, back-probe the harness: the reference pin should show 5 V and the ground pin should show less than 0.1 V.
  4. 4
    Measure sensor resistance at known temperatures against the manufacturer's specification chart (typical: ~2.5 kΩ at 20 °C, ~300 Ω at 80 °C); replace the sensor if resistance is open or out of spec.
  5. 5
    Check coolant level and purge any air pockets; a low system can expose the sensor and cause intermittent or out-of-range readings.
  6. 6
    Clear the code, warm the engine to operating temperature, and rescan to confirm the fault has been resolved.

Related powertrain codes

Frequently asked questions

Can low coolant level trigger P0115?

Yes. If the coolant level is low enough that the sensor tip is exposed to air, the sensor resistance changes unpredictably, and the voltage can drift outside the ECM's acceptable window.

Will P0115 cause the engine to overheat?

Potentially, if the ECM loses the temperature signal it uses to activate the electric cooling fan. Always verify fan operation after diagnosing P0115.

How do I test an ECT sensor with a multimeter?

Measure resistance across the two sensor terminals. At room temperature (~20 °C) expect roughly 2–3 kΩ; at normal operating temperature (~90 °C) it should drop to around 200–300 Ω. An open reading (infinite resistance) or a dead short means the sensor needs replacement.

Is P0115 the same as P0116, P0117, or P0118?

No. P0115 flags a general circuit fault (signal outside range). P0116 is a range/performance rationality issue, P0117 is a hard low-voltage fault, and P0118 is a hard high-voltage fault. P0119 covers intermittent signal behaviour.

Disabling P0115 in software

RaceTune can permanently disable P0115 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

Got P0115 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file