P0118 means the engine computer is reading a voltage from the engine coolant temperature sensor that is too high to be real. Because of how the sensor works, a high voltage tells the computer the engine is impossibly cold, colder than −40 degrees on many engines, even when it is fully warm. The usual cause is a break in the circuit: an unplugged or corroded connector, a broken wire, or a sensor that has failed open. The computer then fuels the engine as if it were frozen, which is why P0118 often brings black smoke, poor fuel economy and hard starting when hot. This article covers what sets P0118, the six real causes in order of likelihood, the jumper test that tells a wiring fault from a failed sensor, and what each repair typically costs.
Start with the table, then read the triage note under it.
| Cause | Telltale Sign | Typical Cost |
|---|---|---|
| Unplugged, loose or corroded connector | Code started after work near the sensor; broken locking tab or green corrosion on the pins | $0 to $150 |
| Failed (open) coolant temperature sensor | Jumpering the harness terminals makes the reading jump to hot | $50 to $250 installed |
| Broken signal or ground wire in the harness | Jumper test changes nothing; damaged harness near the engine | $50 to $300 |
| Open shared sensor ground | Several “high input” codes at once, such as intake air temperature too | $50 to $300 |
| Damage or wrong part from recent repair work | Code appeared straight after thermostat, water pump, head or sensor work | $0 to $250 |
| Engine computer fault | Sensor, connector and wiring all test good | $300 to $1,000 |
Stop driving if there are real signs of overheating (steam, coolant loss, a hot-coolant smell), or if the engine is running very rich: heavy black smoke, a strong fuel smell, or a flashing check engine light. Running rich for long can foul spark plugs and overheat the catalytic converter. If none of those are present, P0118 is usually safe to drive a short distance to a repair, but do not leave it for weeks.
What P0118 Actually Means
The engine coolant temperature (ECT) sensor is a thermistor whose resistance falls as it heats up. The engine computer feeds it a reference voltage, normally 5 volts, through a resistor inside the computer, and reads what is left on the signal wire. Cold sensor, high resistance, high voltage. Hot sensor, low resistance, low voltage. If the circuit is broken anywhere, no current flows and the signal wire floats up to the full reference voltage, which the computer reads as the coldest temperature it can represent.
P0118 is “engine coolant temperature sensor 1 circuit high”. Volkswagen’s 2014 Beetle quick reference specification book gives two concrete versions of the threshold. For the gasoline engines, P0118 sets when the calculated coolant temperature is below −40 °C (which is also −40 °F). For the 2.0L TDI diesel, the same book calls P0118 “Engine Coolant Temperature Sensor Circuit Open or Short to Battery Voltage” and sets it on a signal voltage above 3.25 volts. The name says it plainly. An open circuit is the usual cause, and a short to a voltage source is the less common alternative.
A quick sanity check confirms the fault. With the engine warm, the scan tool’s coolant temperature reading will sit at or near its minimum, typically −40, while the upper radiator hose is too hot to hold. No real engine reads that, so the reading is electrical.
The Jumper Test: Sensor or Wiring?
This is the test that decides what to buy. P0117, the opposite code, is diagnosed by unplugging the sensor. P0118 is diagnosed the other way round, by completing the circuit without the sensor.
- With the ignition on and a scan tool showing live coolant temperature, unplug the sensor. The reading should stay at or drop to its coldest value, because the circuit is now open.
- Connect a fused jumper wire between the two terminals of the harness connector, not the sensor. Those terminals are the signal wire and the sensor ground, so bridging them does what a very hot sensor does. Never jumper either terminal to battery positive.
- If the reading jumps to its hottest value, the wiring and the computer are fine, and the fault is the sensor or its own connector terminals.
- If the reading does not move, the break is in the harness, in either the signal wire or the ground wire, or in the computer. A new sensor will not fix it.
Clear the codes afterwards, because the jumper will briefly set P0117.
Symptoms You May Notice Alongside the Code
A computer that believes the engine is frozen adds cold-start enrichment all the time. The results are the classic rich-running symptoms: black smoke, a fuel smell from the exhaust, poor fuel economy, a rough or lumpy warm idle, and spark plugs that foul. A warm engine flooded with extra fuel is also hard to restart, which is one of the causes covered in the site’s article on a car that is sluggish to start when hot. Many vehicles also run the radiator fan continuously as a failsafe when the coolant signal cannot be trusted. On cars whose gauge uses the same sensor, the gauge may sit at cold, or behave like the erratic gauge covered in why a temperature gauge goes up and down. Fuel trim or rich codes may appear alongside.
1. Unplugged, Loose or Corroded Connector
An open circuit is the textbook cause of P0118, and the easiest place for a circuit to open is the connector. The sensor sits in a hot, vibrating spot, often near the thermostat housing or cylinder head, where plastic locking tabs go brittle and snap. A connector that is not fully latched can work loose, and terminals corroded by coolant seepage or washing water lose contact. Unplug it, inspect both halves for green or white corrosion, spread or pushed-back terminals and a broken tab, then reconnect it firmly and see whether the reading returns to a believable temperature.
2. Failed (Open) Coolant Temperature Sensor
A sensor can fail open internally, reading infinite resistance. The jumper test points here when the reading jumps to hot with the harness terminals bridged. To confirm, measure the sensor’s resistance across its two pins and compare it with the temperature-resistance chart for your engine. As a general pattern, a healthy sensor reads thousands of ohms cold and a few hundred ohms hot. An open reading condemns it.
Many engines have more than one coolant temperature sensor or a separate gauge sender, so confirm which physical sensor is “sensor 1” for your engine. The site’s article on the coolant temperature sensor’s purpose and location covers where it usually sits.
3. Broken Signal or Ground Wire in the Harness
If the jumper test changes nothing, one of the two wires between the connector and the computer is broken, or a terminal at the computer end has lost contact. Heat-hardened insulation cracks, and wires fatigue where the harness flexes with engine movement. With the sensor and computer both disconnected, check each wire for continuity end to end. A reading of open, or of more than a few ohms, locates the break.
Harness faults on this circuit are documented. Ford’s special service message 48991 for 2015 to 2020 vehicles with the 2.7L EcoBoost, including the F-150, Edge and Fusion, lists P0118 among codes, alongside P0116, P0117, P0119, P0128, P0217 and knock sensor code P0330, that may set with a check engine light and/or an engine coolant over-temperature warning. It attributes them to the coolant temperature sensor or knock sensor wiring harness and corrects them by replacing the coolant temperature sensor and the knock sensor.
4. Open Shared Sensor Ground
On many engines several sensors share a common sensor ground (sometimes called the low reference or signal return), including the coolant temperature, intake air temperature and throttle position sensors. If that shared ground opens, every thermistor sensor on it reads as an open circuit at once. The signature is P0118 arriving together with other circuit-high codes, most often P0113 for the intake air temperature sensor. Several sensors almost never fail together, so treat a cluster like this as one wiring fault and trace the shared ground and its connection at the computer or engine block.
5. Damage or Wrong Part From Recent Repair Work
Thermostat, water pump, cylinder head and intake manifold jobs all happen right next to this sensor. A connector left unplugged, a tab snapped while releasing it, or a harness trapped under a housing will set P0118 on the first drive. A replacement sensor with the wrong resistance curve for the engine can also produce readings far outside what the computer expects. If the code appeared straight after a repair, start with that work.
6. Engine Computer Fault
Rare, and the last conclusion. If the sensor tests good, both wires have continuity, the shared ground is intact and the jumper test still does not move the reading, the input circuit inside the computer may have failed. Check first for a service bulletin or software update, and remember that a replacement computer normally needs programming to the vehicle.
Diagnostic Order
- Record every stored code. P0118 with other circuit-high codes points to a shared ground.
- Compare the live reading with reality. A warm engine reading about −40 confirms an electrical fault.
- Inspect and reseat the connector, checking for corrosion, damaged terminals and a broken tab.
- Do the jumper test at the harness connector. Reading jumps to hot: sensor. No change: wiring or computer.
- Measure sensor resistance against the chart, or check each wire’s continuity with both ends disconnected.
- Replace the sensor only once the test points at it.
After fitting a sensor, top up and bleed the cooling system. Removing a sensor from a coolant passage lets coolant out, and an air pocket around the new one can cause erratic readings and a fresh code.
Repair Cost
Typical costs, labelled as typical because sensor location varies a great deal between engines:
- Reseating or cleaning the connector: nothing, if done during diagnosis.
- Connector replacement: roughly $20 to $150.
- Coolant temperature sensor: roughly $50 to $250 installed, including a coolant top-up. Sensors under the intake manifold on some V engines cost more in labour.
- Wiring or shared ground repair: roughly $50 to $300.
- Engine computer: rarely needed; roughly $300 to $1,000 including programming.
Leaving it can add to the bill. Spark plugs fouled by the rich mixture may need replacing, and prolonged rich running can overheat and damage the catalytic converter.
Can You Keep Driving With a P0118 Code?
A short distance, yes, but fix it within days rather than weeks. P0118 is harder on the engine than its opposite, P0117. A computer that thinks the engine is hot leans out cold starts. One that thinks it is frozen keeps adding fuel all the time, which washes oil off the cylinder walls, fouls plugs and sends unburned fuel into the catalytic converter. Fuel economy suffers the whole time.
The coolant temperature signal also stops being a reliable overheating warning while the code is stored, so watch the coolant level and look for real signs of overheating rather than trusting the gauge.
Related Codes
- P0117: the opposite fault on the same sensor. The voltage is too low, so the computer reads an impossibly hot engine, usually from a short to ground. It is diagnosed with the unplug test rather than the jumper test. See the P0117 guide.
- P0116: coolant temperature sensor range or performance. The signal is electrically plausible but does not behave as expected, for example it does not rise as the engine warms.
- P0115 and P0119: a general circuit fault and an intermittent circuit fault on the same sensor.
- P0113: intake air temperature sensor circuit high, the same open-circuit logic on a different sensor. Together with P0118 it points to a shared sensor ground.
- P0073: ambient air temperature sensor circuit high, the same open-circuit logic on the outside air sensor, which often reads -40 and can lock out the A/C. See the P0073 guide.
- P0128: coolant temperature below thermostat regulating temperature. It is judged from this sensor’s reading, so fix P0118 before chasing the thermostat. See the P0128 guide.
Frequently Asked Questions
What does the P0118 code mean?
P0118 means the engine computer is reading a voltage from the coolant temperature sensor that is too high to be real, which it interprets as an impossibly cold engine, often around -40 degrees. The usual cause is an open circuit, such as an unplugged or corroded connector, a broken wire or a sensor that has failed open.
Is it safe to drive with a P0118 code?
A short distance, yes, but fix it within days. The computer thinks the engine is frozen and keeps adding fuel, which can foul spark plugs, wash oil off the cylinder walls and overheat the catalytic converter. Stop driving if there is heavy black smoke, a strong fuel smell or real signs of overheating.
How do I know if it is the sensor or the wiring?
Unplug the sensor, then bridge the two terminals of the harness connector with a fused jumper wire while watching coolant temperature on a scan tool. If the reading jumps to hot, the wiring is fine and the sensor has failed. If it does not move, the break is in the wiring or the computer.
Can P0118 cause black smoke and bad fuel economy?
Yes. Believing the engine is extremely cold, the computer adds cold-start enrichment all the time. Black smoke, a fuel smell, poor economy, a rough warm idle and hard hot starts are all typical.
What is the difference between P0118 and P0117?
They are opposites on the same sensor. P0118 means the voltage is too high, so the computer reads an impossibly cold engine, usually an open circuit. P0117 means it is too low, so it reads an impossibly hot engine, usually a short to ground.
How much does it cost to fix P0118?
Often nothing if the connector just needs reseating, roughly $20 to $150 for a new connector, $50 to $250 installed for a coolant temperature sensor and $50 to $300 for a wiring repair. An engine computer is rarely the cause.
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