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P0118: Engine Coolant Temperature (ECT) Sensor: Circuit High Input

Moderate

Quick answer

P0118 means the engine coolant temperature sensor’s signal voltage is stuck high, which the computer reads as an impossibly cold engine, typically -40°. In practice that is an open circuit: an unplugged or corroded connector, a broken wire, or a sensor that failed open. The engine runs rich and the fans may stay on. Check the connector and wiring before buying a sensor.

What it means

The coolant temperature sensor is a thermistor: a resistor whose value drops as the coolant warms up. The engine computer feeds it a 5-volt reference through a pull-up resistor and reads the voltage that comes back. Cold engine, high resistance, high voltage. Hot engine, low resistance, low voltage. P0118 sets when that voltage sits at or near the 5-volt ceiling for longer than the computer allows, which corresponds to a temperature no running engine can have.

That is why the live-data reading on a P0118 car is so often exactly -40 (the same number in Celsius and Fahrenheit): it is the value the computer assigns when the circuit is open. A wire that broke, a connector that backed out, or a sensor element that cracked all look identical from the computer’s seat. A short to voltage can do it too, but it is rare. The mirror-image code, P0117, is the signal stuck low (a short to ground) and reads impossibly hot.

While the signal is untrusted the computer substitutes a default temperature and often turns the cooling fans on as a safety measure. The engine keeps running, but it is fueling for a frozen morning that never ends: hard hot starts, a rich fuel smell, worse economy, and on some vehicles no temperature reading on the dash at all.

P0118 symptoms: what you'll notice

  • Cooling fans running at full speed from the moment the key turns, even on a cold engine: the computer’s failsafe when it cannot see coolant temperature.
  • Hard starting when the engine is already warm, sometimes with black smoke, because the computer is adding cold-start fuel to a hot engine.
  • A rich fuel smell at idle and noticeably worse fuel economy.
  • The temperature gauge reading cold or not moving at all on vehicles where the gauge shares the same sensor; on many cars the gauge has its own sender and still reads normally.
  • Live data showing coolant temperature at -40 with the engine warm: the fingerprint of an open circuit.

Common causes

Ordered from most to least likely.

  1. 1.

    Unplugged, loose, or backed-out connector at the sensor

    The most common answer after any recent work near the thermostat housing or intake. A terminal pushed out of the plug looks connected and reads open.

  2. 2.

    Broken or chafed wire in the sensor harness

    The two wires run past hot, vibrating metal. Heat-hardened insulation cracks, and a wire can break inside its jacket with no visible damage.

  3. 3.

    Corroded terminals raising the circuit’s resistance

    Green or white crust inside the plug adds resistance, which the computer reads as a colder engine, then as open. Common where coolant has leaked onto the connector.

  4. 4.

    Sensor failed open internally

    Age, thermal cycling, or a past overheat cracks the thermistor element. The sensor looks fine and reads infinite resistance.

  5. 5.

    Lost sensor ground at the computer side

    If the intake air temperature or throttle sensor faults at the same time, suspect a shared ground or reference circuit rather than three coincidences.

  6. 6.

    Short to voltage or a failed computer input

    Rare. A harness melted against an ignition feed, or a computer pin damaged by a jump-start mistake. Rule out everything above first.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Read live data first

    With the engine warm, look at the coolant temperature PID. A reading of -40 confirms an open circuit and tells you this is a wiring-or-sensor problem, not a plausibility problem. Compare it with intake air temperature: on a cold engine the two should match within a few degrees.

  2. 2 Inspect the connector under good light

    Unplug the sensor. Look for corrosion, coolant residue, a spread or backed-out terminal, and a broken locking tab that lets the plug work loose. Clean with contact cleaner, straighten pins, reseat firmly, and recheck live data. This free step closes a large share of P0118 cases.

  3. 3 Jumper the two terminals in the harness plug

    Key on, sensor unplugged, bridge the two harness terminals with a paper clip or jumper wire. The reading should swing to the hot extreme (and may set P0117). If it does, the harness and the computer are proven good and the sensor is the fault. If the reading stays at -40, the open is in the wiring or the computer.

  4. 4 Back-probe the reference and ground

    Key on, plug disconnected: one terminal should show close to 5 volts, and the other should read near 0 ohms to the battery negative. A missing 5 volts or a bad ground means a broken wire between the plug and the computer. Trace it, watching for chafe points near the exhaust manifold, the thermostat housing, and any harness clip.

  5. 5 Measure the sensor’s resistance

    With the sensor unplugged, read across its two pins. Most sensors read roughly 2,000 to 3,000 ohms at room temperature and a few hundred ohms at operating temperature; your service data has the exact table. Infinite resistance is a failed sensor. A value in range means the sensor is fine and you go back to the harness.

  6. 6 Wiggle-test an intermittent

    If the code comes and goes, run the engine with live data on screen and flex the harness along its route while tapping the connector. A reading that jumps to -40 when you touch a spot has just located the break for you.

  7. 7 Replace with an OEM-quality sensor and verify the warm-up curve

    After the fix, watch the reading climb smoothly from ambient to about 85 to 105°C with no jumps. Bargain sensors are a common cause of a P0118 that returns weeks later; the part is cheap enough to buy right the first time.

Known P0118 patterns by make

Brand-specific failure patterns from the field: find your make before parts-shopping.

Ford 2000–2011 Focus, Escape, Ranger and other 2.0L/2.3L/3.0L applications

The sensor sits in the plastic thermostat housing or the cylinder head right next to it, and those housings crack, leak and cook the connector. A coolant-soaked plug corrodes the two terminals and reads open. When you fix a P0118 here, inspect the housing for seepage and replace the pigtail if the terminals are green; a new sensor alone comes back.

Chrysler / Dodge / Jeep 2002–2012 3.7L V6, 4.7L V8 and 5.7L Hemi trucks and SUVs

The coolant temperature sensor threads into the front of the cylinder head or the thermostat housing area and its connector lock tab breaks easily, so the plug works loose over time and the pins spread. A P0118 with fans roaring on a warm engine is the typical presentation. Reseat and lock the connector first, then test the sensor before replacing it.

Toyota / Lexus 1998–2012 Camry, Sienna, Highlander, RAV4, ES and RX with the 1MZ, 3MZ and 2GR V6

An open coolant temperature circuit on these sets P0118 almost immediately and commands the electric fans to high as a failsafe, which is often the first thing the owner notices. The harness runs close to the intake plenum and the rear bank, a spot where insulation chafes and connectors get knocked during spark plug jobs. Check the plug and harness before the sensor.

Parts & tools you may need

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Related codes

Frequently asked questions

What does code P0118 mean?
P0118 means the engine coolant temperature sensor circuit is reading high: the signal voltage is stuck near 5 volts, which the computer interprets as an impossibly cold engine. It is almost always an open circuit (connector, wire, or sensor), and it is moderately serious: you can drive gently, but fix it soon.
Can I drive with P0118?
Short term, yes. The engine runs on a default temperature, which means rich fueling, hard hot starts, and fans that run constantly. The risks are fouled spark plugs and extra strain on the catalytic converter, plus the fact that a real overheat could hide behind a gauge that is not reading. Fix it within days, not months.
Why does my scanner show -40 for coolant temperature?
Because -40 is what the computer reports when the sensor circuit is open. It is not a measurement, it is a placeholder. That single number tells you to look for a disconnected plug, a broken wire, or a sensor that failed open.
Is it the sensor or the wiring?
The jumper test answers it in one minute. Unplug the sensor, bridge the two harness terminals with the key on, and watch live data. If the reading jumps to the hot extreme, the wiring and computer are fine and the sensor is bad. If it stays at -40, the problem is upstream in the harness.
Why are my radiator fans running all the time?
When the computer cannot trust the coolant temperature signal, it turns the fans on as insurance against overheating. The fans will go back to normal control as soon as the circuit is repaired and the code is cleared.
How much does it cost to fix P0118?
A coolant temperature sensor is $15 to $60 and takes 15 to 45 minutes on most engines, so $60 to $150 at a shop. A connector pigtail is $15 to $40. A harness repair or a corroded splice runs $100 to $250 depending on how buried it is. Most P0118 cases end at the connector or the sensor.
What is the difference between P0116, P0117, P0118 and P0125?
P0117 is the signal stuck low (short to ground, reads too hot). P0118 is the signal stuck high (open circuit, reads too cold). P0116 is a plausibility code: the signal is electrically fine but its value or behavior does not make sense, often a thermostat. P0125 means the coolant never got warm enough for closed-loop fuel control, again usually a thermostat.
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