Engine Coolant Temperature Sensor 1 Circuit

What it means

The engine coolant temperature (ECT) sensor is an NTC thermistor; the ECU reads its voltage via a 5 V pull-up and uses the result for cold-start enrichment, idle speed, ignition timing, fan control and closed-loop enable. P0115 sets when the signal is outside the valid range (roughly 0.1-4.9 V) or fails a plausibility check, such as not rising after several minutes of running or disagreeing with the IAT sensor after a cold soak. The ECU then runs on a modelled temperature.

Symptoms

• Check engine light; temperature gauge may read wrong or drop to zero on cars using the same sensor
• Hard cold start or hard hot start depending on the substitute value
• Black smoke and high consumption if the ECU assumes a cold engine
• Radiator fan running constantly as a failsafe
• Rough idle and no closed-loop lambda control, failing emissions test

Common causes

1. ECT sensor failed open, shorted or drifted out of specification
2. Corroded terminals or coolant inside the connector
3. Broken or chafed wiring, especially near the thermostat housing
4. Low coolant level leaving the sensor tip in air so it reads slowly (plausibility fault)
5. Thermostat stuck open so the engine never reaches operating temperature
6. ECU input fault (rare)

How to diagnose

1. Read ECT, IAT and ambient in live data on a cold engine (after 6 or more hours parked): all three should agree within 5 °C; -40 °C or +140 °C means an open or short
2. Unplug and measure the sensor: typical two-wire NTC values are 5.0-6.5 kohm at 0 °C, 2.2-2.8 kohm at 20 °C, 0.9-1.2 kohm at 40 °C, 0.28-0.36 kohm at 80 °C, 0.18-0.22 kohm at 100 °C (verify against OEM table)
3. Key on, check the connector: 5 V on the signal wire with the sensor unplugged, under 0.1 V on the ground wire
4. Jumper the pins: scan tool should jump to maximum (about 140 °C); open: -40 °C. Both OK means the sensor is at fault
5. Run the engine from cold and watch ECT climb smoothly to 85-105 °C in 5-10 minutes; a plateau at 60-70 °C suggests a stuck-open thermostat, not the sensor
6. Check coolant level and bleed the system if the sensor sits in an air pocket

Symptoms linked to this code

• Engine idle surging up and down

Terms to know

Related codes

Nearby codes in this group

• P0114Intake Air Temperature Sensor 1 Circuit Intermittent
• P0116Engine Coolant Temperature Circuit Range/Performance
• P0113Intake Air Temperature Sensor 1 Circuit High (Bank 1)
• P0117Engine Coolant Temperature Sensor 1 Circuit Low
• P0112Intake Air Temperature Sensor 1 Circuit Low (Bank 1)
• P0118Engine Coolant Temperature Sensor 1 Circuit High
• P0111Intake Air Temperature Sensor 1 Circuit Range/Performance (Bank 1)
• P0119Engine Coolant Temperature Circuit Intermittent