P0118: Engine Coolant Temperature Sensor Circuit High Input

What the ECT Sensor Does

The ECT sensor is a 2-wire NTC thermistor threaded into the engine block, cylinder head, or thermostat housing. It measures coolant temperature and sends the information to the PCM. This single sensor input affects:

• Fuel delivery: Cold engines need a richer mixture. The PCM uses ECT to calculate cold start enrichment, warm-up enrichment, and closed-loop entry.
• Ignition timing: Cold engines get advanced timing for smoother idle.
• Idle speed: Cold idle RPM is higher than warm idle. The PCM controls this based on ECT.
• Cooling fan operation: The PCM turns fans on and off based on ECT reading.
• Transmission shift points: Some transmissions delay converter clutch application until the engine reaches operating temperature.
• Emissions monitors: Most OBD-II monitors will not run until ECT reaches a specific temperature — a stuck-cold ECT reading keeps monitors from completing.

Because it touches so many systems, a failed ECT sensor can cause a cascade of symptoms that look like multiple unrelated problems.

Why a Bad ECT Causes So Many Problems

If the ECT circuit is open (P0118), the PCM thinks the engine is at -40°F. It will:

• Command maximum cold start enrichment — the engine runs extremely rich
• Command high idle speed — 1,500+ RPM when the engine is actually warm
• Delay or prevent closed-loop fuel control — fuel trims are locked out
• Prevent emissions monitors from running — inspection failure
• Potentially disable the cooling fans — the PCM thinks the engine is freezing

A single $12 sensor can make a vehicle nearly undriveable. The symptoms often mimic a fuel system problem, an idle control problem, or an emissions failure — and techs who do not check the basics first will chase those rabbit holes for hours.

The Unplug and Jumper Test

Same technique as any NTC temperature sensor:

1. Unplug the ECT sensor — the scan tool should show -40°F or the maximum cold value. If it already showed this before you unplugged it, the circuit was already open. Check the connector for spread or corroded pins.
2. Jumper the two pins at the harness connector (signal to ground) — the scan tool should jump to maximum hot (260°F+, near 0V). If it does, the wiring and PCM are good. Replace the sensor.
3. If jumpering has no effect — the signal or ground wire is open between the connector and the PCM. Check for 5V reference at the signal pin with the sensor unplugged. If 5V is missing, trace the wire.

This 2-minute test eliminates the PCM and wiring from the equation. You will know whether to buy a $12 sensor or spend an hour tracing wiring before you ever pick up a part.

Cold Soak Comparison

After the vehicle has sat overnight, connect a scan tool before starting the engine. ECT and IAT should read within 5-10°F of ambient temperature. If ECT reads -40°F on a 70°F morning, you have an open circuit. If ECT reads 200°F on a cold engine, you have a shorted circuit or a wrong sensor installed (some aftermarket sensors have the wrong resistance curve for the application).

This technique also catches thermostats stuck open. If the ECT never reaches 195-210°F during a 15-minute drive in warm weather, the thermostat may be stuck open. The PCM will keep trying to warm up the engine and may set a P0128 (thermostat performance below threshold).

Step-by-Step Diagnosis

Step 1: Cold Soak Reading

Check ECT on a cold engine. It should match ambient. If it reads -40°F, you have your answer — open circuit.

Step 2: Visual Inspection

Check the ECT connector for coolant contamination (a leaking sensor or housing will push coolant into the connector and corrode the pins), corrosion, and wire damage. Coolant is electrically conductive and will cause erratic readings before causing an open circuit.

Step 3: Unplug and Jumper Test

Confirm whether the problem is the sensor or the wiring as described above.

Step 4: Sensor Resistance Check

Remove the sensor. Measure resistance at room temperature — it should be approximately 2,200-3,000 ohms at 68°F. OL (open line) means the sensor is bad. If you want to fully test it, put the sensor in a pot of water with a thermometer and measure resistance as the water heats up. Resistance should decrease smoothly without any dropouts or jumps. A dropout is an intermittent open that will cause random P0118 codes.

Step 5: Check for Correct Sensor

Aftermarket ECT sensors sometimes have the wrong resistance curve. If you just replaced the sensor and the temperature reading is still wrong (not -40°F, but consistently off by 20-30°F), cross-reference the part number. The sensor might physically fit but have a different NTC curve designed for a different manufacturer.

Common Failure Points

• GM trucks (4.3L, 5.3L, 6.0L): Coolant contamination at the connector is the #1 issue. The sensor threads into the thermostat housing or intake manifold, and slow coolant seepage pushes fluid into the electrical connector. Clean and dry the connector, apply dielectric grease, and replace the sensor.
• Ford (4.6L, 5.4L): The ECT sensor on some applications is located in a position where it is exposed to road splash. Corrosion builds up on the connector over time.
• Honda/Acura: Single-wire gauge sensors are sometimes confused with the 2-wire PCM sensor. If you replace the wrong one, the code will still be there.
• Chrysler/Jeep: The 3.6L Pentastar has a known issue with the ECT sensor connector corroding internally. Even if the sensor tests good, the connector may need to be repaired or replaced.

Repair Costs