P0118 means the engine coolant temperature sensor signal is stuck high — about 4.6-5 V — so the computer reads impossibly cold coolant, often -40°F. An unplugged connector, corroded terminal, broken wire or failed $20-$60 sensor is the usual cause. Expect rich running, fans running constantly and hard hot starts; most repairs cost $0-$250.
- P0118 is set when the sensor signal rises to about 4.6-5 V, which most scan tools display as -40°F (-40°C).
- Unplugging the sensor and bridging the harness terminals should swing the reading to hot; if it does, the sensor is bad.
- Most P0118 repairs cost $0-$250; a replacement sensor part is typically $20-$60.
- The computer adds cold-start fuel, so the engine runs rich and many vehicles run the fans all the time.
- What Does P0118 Mean?
- What Are the Symptoms of P0118?
- What Causes P0118?
- How Do You Diagnose P0118?
- How Do You Fix P0118 and What Does It Cost?
- Why Does P0118 Make the Engine Run Rich?
- How Is P0118 Different From P0117 and P0128?
- Can I Drive With P0118?
- When Should You See a Mechanic?
- Frequently Asked Questions
- Related Guides
What Does P0118 Mean?
P0118 is the generic OBD-II code for “Engine Coolant Temperature Sensor 1 Circuit High”. The engine computer has seen a coolant sensor signal voltage higher than any real coolant temperature could produce, close to its 5-volt reference. It reads that as impossibly cold coolant, so the fault is usually electrical, not thermal.
The engine coolant temperature (ECT) sensor is a negative temperature coefficient (NTC) thermistor: its resistance falls as it warms. The computer feeds it 5 volts through an internal resistor and reads what is left. A cold sensor has high resistance, so the signal sits high; a hot sensor pulls it low. If the circuit opens — an unplugged connector, a broken wire, a burnt-out sensor — nothing pulls the voltage down, and the signal rises to about 4.6-5 V. Many scan tools then show a coolant temperature of -40°F (-40°C), the bottom of the scale.
| Coolant temperature | Typical sensor resistance | Typical signal voltage |
|---|---|---|
| 68°F (20°C), cold engine | about 2,300-3,500 Ω | about 3-4 V |
| 176°F (80°C) | about 300-340 Ω | about 1 V |
| 212°F (100°C), fully warm | about 180-200 Ω | about 0.5-0.8 V |
| Open circuit (P0118) | infinite (meter shows OL) | about 4.6-5 V, reads -40°F |
These are typical values from common GM and Japanese sensors; curves vary by manufacturer, so use the service manual figure for your engine. The “Sensor 1” wording matters on engines with two coolant sensors; P0118 refers to the main sensor the computer uses for fuelling.
What Are the Symptoms of P0118?
P0118 symptoms come from the engine computer believing the engine is freezing cold even when it is fully warm. It adds cold-start fuel, may raise the idle and, on many vehicles, switches the cooling fans on permanently as a safety default because it can no longer judge the real temperature.
- Check engine light on, often soon after starting.
- Cooling fans running all the time, even on a cold engine or with the key just on.
- Rich running when warm: rough idle, a fuel smell, sooty tailpipe or black smoke from the exhaust.
- Hard starting when the engine is hot — the extra cold-start fuel floods it.
- Higher idle speed that never settles down.
- Worse fuel economy, sometimes a fuel-trim code such as P0172 (system too rich).
- A temperature gauge stuck at cold on cars where the gauge shares the sensor — it can then hide a real overheat, so watch for steam, a coolant smell or the red temperature warning light.
What Causes P0118?
P0118 is caused by an open or high-resistance coolant sensor circuit far more often than by a faulty sensor or computer. Anything that breaks the path between the 5-volt reference, the thermistor and its ground return lets the voltage rise. Check the cheap, likely causes first, in this order:
- Connector unplugged or not fully latched — very common after a thermostat, water pump, hose or intake job near the sensor.
- Corroded, coolant-soaked or spread terminals — green corrosion or loose pins add resistance and make the reading jump.
- Failed sensor (open thermistor) — the sensor itself breaks internally; on older sensors this is the most common single cause.
- Broken or chafed signal wire — harness rubbing on a bracket, heat damage near the exhaust manifold, or rodent damage.
- Open sensor ground (return) circuit — this return is often shared, so an intake air temperature code such as P0113 at the same time points to the shared wire or connector.
- Engine computer fault — rare; confirm everything above before considering it.
Low coolant can make readings erratic but rarely produces a true circuit-high voltage.
How Do You Diagnose P0118?
Diagnosing P0118 takes about 30-60 minutes with a scan tool that shows live data and a basic digital multimeter. The goal is to prove whether the sensor or the wiring is open before buying anything.
1. Read live coolant temperature (5 minutes)
On an engine that has sat overnight, the coolant reading should be within about 10°F (5°C) of the outside air temperature and intake air temperature. A reading of -40°F confirms an open circuit.
2. Wiggle test the connector and harness (5 minutes)
With the key on and live data showing, gently move the connector and harness along its route. A reading that flicks to -40°F means a loose terminal or broken wire, not a bad sensor.
3. Jumper test at the connector (10 minutes)
Unplug the sensor, then bridge the two harness terminals with a fused jumper or the test lead your service manual specifies. The reading should swing to its hottest value (often 250°F or more) and P0117 may set. If it swings, the wiring and computer are fine and the sensor is open. If it stays at -40°F, the fault is in the wiring.
4. Check reference and ground (10-15 minutes)
With the sensor unplugged and key on, the reference terminal should show about 5 V to ground. Key off, measure the ground terminal to battery negative: it should be under about 5 Ω.
5. Measure the sensor itself (10 minutes)
Measure across the sensor pins and compare with the temperature table above or your service figure. OL (open) or a value far out of range at a known temperature condemns the sensor.
How Do You Fix P0118 and What Does It Cost?
Fixing P0118 means repairing whichever part of the circuit is open — often just a connector, sometimes a $20-$60 sensor. Typical US costs, October 2026 — varies by vehicle and region:
| Confirmed cause | Fix | Typical cost (parts + labour) |
|---|---|---|
| Unplugged or loose connector | Reconnect and latch | $0-$100 |
| Corroded terminals or pigtail | Clean terminals or splice a new pigtail | $30-$200 |
| Failed ECT sensor | Replace sensor (part $20-$60), top up and bleed coolant | $80-$250 |
| Broken signal or ground wire | Locate and repair harness | $100-$400 |
| Engine computer | Repair or replace and program | $400-$1,500 |
After the repair, clear the code and confirm the live reading climbs smoothly from ambient to about 195-220°F (90-105°C) as the engine warms.
What doesn’t work: clearing the code without a repair (it returns within seconds of starting), replacing the thermostat (that is a P0128 fix, not a circuit fix), and buying an engine computer before checking the connector.
Why Does P0118 Make the Engine Run Rich?
P0118 makes the engine run rich because the computer believes the coolant is extremely cold. A cold engine needs extra fuel to start and run, so the computer lengthens injector pulses, raises idle and may retard timing. On a warm engine that extra fuel is not needed, and the mixture turns rich.
Many computers substitute a default temperature once P0118 is stored, but the result is still higher fuel use, fouled spark plugs if ignored for months, a strong fuel smell and extra heat in the catalytic converter. A converter costs far more than any sensor.
How Is P0118 Different From P0117 and P0128?
P0118, P0117 and P0128 all involve coolant temperature but describe different faults. P0118 and P0117 are circuit codes — the signal voltage is outside any possible range — while P0128 is a performance code meaning the engine takes too long to warm up, usually because of a stuck-open thermostat.
| Code | Signal | Computer believes | Usual cause |
|---|---|---|---|
| P0118 | Too high (about 4.6-5 V) | Extremely cold coolant | Open circuit, unplugged or failed sensor |
| P0117 | Too low (near 0 V) | Extremely hot coolant | Short to ground, shorted sensor |
| P0128 | Plausible but slow to rise | Engine not reaching temperature | Thermostat stuck open |
The same logic applies to the intake air temperature sensor: P0113 is its circuit-high code, and P0113 with P0118 together usually means a shared ground or connector.
Can I Drive With P0118?
You can usually drive with P0118 for a few days to reach a repair, as long as the real engine temperature is normal and the engine runs acceptably. Expect higher fuel use and constant fan noise. Stop if you see steam, smell coolant or the engine stumbles badly, and do not leave it for weeks.
When Should You See a Mechanic?
See a mechanic for P0118 when the jumper test points to wiring you cannot trace, the code returns after a new sensor, other sensor codes appear together, or the car shows any sign of genuine overheating. Hidden harness breaks and shared-ground faults need wiring diagrams and patience more than parts. More coolant and sensor codes are in our trouble codes section.
Short trips to a repair are usually fine if the real temperature is normal. Stop if you see steam, smell coolant or the engine stumbles.
Have the car checked if the code returns after a sensor replacement, P0118 appears with other sensor codes, or the engine shows real overheating signs such as steam or a coolant smell. Never open the radiator or expansion tank cap on a hot engine — pressurised coolant can cause severe burns. Let it cool for at least 30 minutes, and keep hands away from fans, which may start on their own.
Frequently Asked Questions
Does P0118 mean my engine is too cold?
Usually not. P0118 means the coolant sensor circuit reads an impossibly cold temperature, typically -40°F, because the signal voltage is stuck high. The engine itself is normally at its usual temperature. Compare the scan-tool reading with the real temperature of the thermostat housing before suspecting the engine.
Can a bad coolant temperature sensor cause high idle?
Yes. When the sensor circuit is open, the computer believes the engine is cold and holds a raised cold-start idle, often 1,000-1,500 rpm, even when fully warm. The idle should drop to normal once the circuit is repaired and the code cleared, assuming no vacuum leak or throttle body fault.
Why are my radiator fans always on with P0118?
Many engine computers switch the cooling fans on permanently once a coolant sensor circuit fault is stored. Without a believable temperature reading, running the fans is the safest default against overheating. The fans should behave normally again after the circuit is repaired and the code cleared.
How do I reset a P0118 code?
Repair the open circuit first, then clear the code with a scan tool or let the computer clear it after several clean drive cycles. Disconnecting the battery also clears it but resets radio and idle learning. If the circuit is still open, P0118 returns within seconds of starting the engine.
Can I drive long distances with P0118?
Short trips to a repair are usually fine if the real temperature is normal, but long drives are a poor idea. The rich mixture wastes fuel and heats the catalytic converter, and on cars whose gauge shares the sensor you lose your overheating warning. Fix the circuit within a few days.
Is the coolant temperature sensor the same as the temperature gauge sender?
On many modern cars one sensor serves both the engine computer and the gauge, but some vehicles use a separate sender for the gauge. If your gauge works normally while P0118 is stored, the car probably has two sensors, and P0118 refers to the one the engine computer uses.
