Engine Coolant Temperature Sensor 1 Circuit

What it means

The engine computer (ECU) has detected an electrical problem in the engine coolant temperature (ECT) sensor circuit. The engine coolant temperature (ECT) sensor tells the ECU how hot the engine is, which affects fuel mixture, idle speed, cooling fans and the temperature gauge. The signal is missing or outside the range the ECU will accept.

Symptoms

• Check engine light on
• Temperature gauge reading wrong or not moving
• Cooling fans running all the time
• Hard starting, especially cold
• Poor fuel economy and black smoke

Probable causes

Most likely first

1. 1Damaged, corroded or unplugged connector
2. 2Open or shorted wiring
3. 3Failed sensor

How to diagnose it

Work through these in order before buying parts

1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
2. 2Inspect the engine coolant temperature (ECT) sensor connector and wiring (in the thermostat housing, cylinder head or intake manifold coolant passage). Look for corrosion, bent or pushed-back pins, oil contamination and chafed insulation.
3. 3Watch the engine coolant temperature (ECT) sensor reading on live data. After a cold soak it should match the outside temperature. With the engine warm it should rise steadily to about 85–105 °C (185–220 °F) and stay there.
4. 4Unplug the sensor and measure its resistance; it should drop smoothly as it warms up (compare with the chart in a service manual). With the key on, the ECU side of the connector should show about 5 volts on the signal wire and a good ground on the other.
5. 5If the wiring checks out, test the sensor itself against the specification in a service manual.
6. 6Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

• Repair the damaged wiring or connector
• Replace the engine coolant temperature (ECT) sensor
• Clear the code and test drive to confirm the repair