Engine Coolant Temperature Circuit High

What P0118 means

The ECT sensor is a negative-temperature-coefficient thermistor connected in series with an internal pull-up resistor inside the ECM. When coolant temperature rises, sensor resistance falls and the ECM signal-pin voltage drops. If the signal circuit opens — due to a broken wire, a disconnected connector, or a failed sensor with an open-circuit element — the ECM sees only its own pull-up voltage (approximately 5 V). P0118 is stored after the ECT sensor voltage remains above 4.91 V for 0.5 seconds or more (1-trip detection logic). A scan tool will display -40 °C while this fault is active, which is the ECM's lowest mapped temperature value for an open circuit. As with P0117, the ECM enters fail-safe mode and substitutes 80 °C for all calculations. The monitor runs continuously with no other preconditions.

Symptoms

• Check engine light on immediately — P0118 is a 1-trip code
• Scan tool shows coolant temperature stuck at -40 °C regardless of actual engine temperature
• Rich running and excessive fuel consumption — the ECM may apply cold-start enrichment indefinitely because it thinks coolant is at -40 °C
• Rough idle on a warm engine that is receiving unnecessary cold-start enrichment
• Fan may not engage correctly — cooling fan activation depends on ECT data that is now substituted

Common causes

• Open circuit in the ECT sensor signal wire between the sensor and ECM — most common cause, especially at the connector
• Failed ECT sensor with an open-circuit thermistor element inside the sensor body
• Disconnected or poorly seated sensor connector — physical disconnection of the two-wire connector produces the same signature as an open circuit
• Corroded or broken terminal at the ECM connector for the THW (coolant temperature signal) input
• Wire break in the harness — particularly in areas subject to flex or vibration near the firewall or at harness-to-bracket contact points

Severity & driving advice

Severity: medium — Fail-safe forces cold-start enrichment on a warm engine, degrading fuel economy and emissions. Not immediately dangerous but impairs fuel control.

Can I drive? Repair soon ? driveable but fuel economy impaired.

Diagnostic approach

1. Confirm scan tool reads -40 °C and cross-check with actual coolant temperature — A scan tool reading fixed at -40 °C (regardless of actual engine warmth) confirms the open-circuit P0118 condition. Contrast: if it reads above 135 °C, the fault is P0117 (short to ground), not an open. Do not confuse the two.
2. Disconnect the ECT sensor and jumper the two connector terminals together — With the sensor connector unplugged, use a jumper wire to bridge terminals 1 and 2 of the harness-side connector. The scan tool should now display above 135 °C. If it does, the harness between the connector and ECM is intact — the sensor itself is the open-circuit source.
3. Check for open in wire harness between sensor and ECM — If jumpering the sensor connector does not raise the scan tool reading above 135 °C, there is an open in the harness. Disconnect the ECM connector and verify resistance below 1 Ω between the sensor connector signal pin and the corresponding ECM THW pin. High resistance or open circuit confirms a broken wire.
4. Verify ECM connector integrity — With the ECT sensor connector unplugged and the ECM connector disconnected, measure resistance on the ECM connector pin for THW to chassis ground — should be 10 kΩ or higher. Also inspect the connector for bent, pushed-back, or corroded terminals. Clean with electrical contact cleaner and re-seat if any corrosion is found.
5. Measure resistance of the ECT sensor itself — With the sensor removed and at ambient temperature (approximately 20 °C / 68 °F), measure resistance across the two sensor terminals. At 20 °C, a healthy ECT sensor typically reads 2–3 kΩ. At 80 °C, expected resistance is approximately 300 Ω. An open circuit (infinite resistance) at any temperature confirms a failed sensor element requiring replacement.
6. Replace the sensor and confirm operation — After replacement, clear codes and start the engine cold. The scan tool temperature should track from a realistic cold value (matching ambient air temperature) upward toward 75–100 °C as the engine warms. A reading that jumps immediately to 80 °C suggests the fail-safe is still active — confirm the connector is fully seated and no codes are stored.

Make & model notes

Toyota: On Toyota trucks and SUVs with the 1GR-FE V6 (FJ Cruiser, 4Runner, Tacoma), the ECT sensor connector is a two-pin black plastic connector mounted on the engine block. The connector retainer clip can crack with age, allowing the connector to back out partially and create an intermittent open that sets P0118 at cold start only — warming may push the connector back in and clear symptoms.

Honda: Honda Civic and Accord models from 2006–2015 occasionally produce P0118 from a broken wire at the harness-to-sensor connector junction. The wire insulation becomes brittle from heat cycling, and the break is often invisible until the harness is flexed.

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