# P0118: Engine Coolant Temperature Sensor 1 Circuit High **Verdict:** Drive to a shop this week Cooling - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $50-$400 - **Urgency:** This week - **Most likely:** Failed ECT sensor or loose connector - **DIY?:** Yes, steps 1-2 ## Quick answer P0118 means the voltage from the engine coolant temperature (ECT) sensor is too high, which the engine computer reads as an extremely cold engine, often about -40 degrees. That almost always means an open circuit, such as an unplugged or corroded connector, a broken wire or a failed sensor. Expect the cooling fans to run constantly and the engine to run rich. ## What does P0118 mean? The ECT sensor is a thermistor threaded into a coolant passage, usually near the thermostat or in the cylinder head. Its resistance is high when the coolant is cold and falls as it warms. The computer feeds it a 5-volt reference and reads the voltage that remains on the signal wire, so a cold engine shows high voltage and a hot engine low voltage. That reading steers warm-up fuel, idle speed and cooling fan control. P0118 sets when the signal sits near the full reference voltage, beyond what even an engine in arctic cold would produce, for a few seconds. That points to a break in the circuit, since an open sensor or wire lets the reference voltage reach the computer unchanged. Many makers set the code quickly, substitute a default temperature and command the cooling fans on. "Sensor 1" is the main ECT sensor. The opposite, low-voltage fault is P0117. ## What are the symptoms of P0118? - Check engine light on - Cooling fans running nonstop - Temperature gauge reading cold or not moving on cars where it shares this sensor - Black smoke, a fuel smell or worse fuel economy from rich running - Rough idle or hard starting once the engine is warm ## What causes P0118? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed (open) coolant temperature sensor** (most common). The thermistor element or its internal connection can break after years of heat cycling, opening the circuit. The scan tool then shows about -40 degrees no matter how warm the engine is. If briefly bridging the two connector terminals with a fused jumper makes the reading swing very hot, the wiring is good and the sensor has failed. 2. **Unplugged, loose or corroded connector** (common). The ECT connector sits in a hot, sometimes wet spot, and its plastic latch can crack and let it back off. Coolant seepage and road moisture corrode the terminals and add resistance. It is also easy to leave unplugged after thermostat, hose or intake work. Reseating the connector and cleaning the pins fixes many P0118 codes. 3. **Broken signal or ground wire** (common). A wire broken inside its insulation, a terminal pushed out of the connector, or an open sensor ground circuit all lift the signal voltage to the top of the range. Heat-hardened harness sections near the thermostat housing and areas disturbed during engine work are prime suspects. A wiggle test while watching live data often reveals intermittent breaks. 4. **Signal wire shorted to a powered circuit** (less common). If the signal wire touches a powered wire in the same harness, the voltage reads high even with a good sensor. This can follow a melted harness or a poor splice repair. With the sensor unplugged and the key on, a voltage well above 5 volts on the signal wire points to a short to another circuit. ## How do you diagnose P0118? 1. Read ECT on a scan tool; about -40 degrees on a warm engine confirms an open circuit. 2. Check that the connector is fully latched, and look for corrosion, coolant and pushed-out pins. 3. With the sensor unplugged, briefly bridge the two terminals with a fused jumper; the reading should jump very hot. 4. If it does not, check the signal and ground wires for breaks between the sensor and the computer. 5. Measure sensor resistance against the maker's chart before replacing it. ## How do you fix P0118, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Reconnect or replace the connector | $50-$200 | Cheapest fix when the latch is broken or the pins are corroded. | | Replace coolant temperature sensor | $100-$350 | Includes a coolant top-up and bleeding air from the system. | | Repair broken wiring | $100-$400 | Cost depends on how long it takes to find the break. | Full price breakdowns: [thermostat replacement cost](/cost/thermostat-replacement), [coolant flush cost](/cost/coolant-flush). ## Is it safe to drive with P0118? > **Drive to a shop this week.** Usually safe for short trips, but the engine may run rich, waste fuel and foul spark plugs over time. Because the temperature gauge may not work, watch for steam or a sweet smell and stop if you see either. Fix it within a week or two. ## Will P0118 fail an emissions test? P0118 keeps the check engine light on, so the vehicle fails an OBD-II emissions test in most states. With the computer thinking the engine is always cold, it may run rich and never run the monitors that need a warm engine. After the repair, drive several full warm-up trips before going to an inspection. ## After the repair: clearing P0118 After the repair, clear the code and watch ECT from a cold start: it should start near outside temperature, climb steadily to the normal operating range, then level off. The fans should cycle normally, the gauge should move, and fuel economy should recover over the next few tanks. No pending code means the fix worked. ## How to prevent P0118 - Check that the ECT connector latch clicks after any cooling system or intake work. - Fix coolant seepage at the thermostat housing before it corrodes nearby terminals. - Replace the coolant on schedule; worn-out coolant corrodes metal parts and sensor tips. - Replace brittle connectors and pigtails instead of taping them, since tape traps moisture. ## Codes often seen with P0118 - [P0117](/codes/p0117): Engine Coolant Temperature Sensor 1 Circuit Low - [P0128](/codes/p0128): Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) - [P0125](/codes/p0125): Insufficient Coolant Temperature for Closed Loop Fuel Control - [P0113](/codes/p0113): Intake Air Temperature Sensor 1 Circuit High (Bank 1) - [P0172](/codes/p0172): System Too Rich (Bank 1) ## Frequently asked questions ### Can I drive with a P0118 code? You can usually drive short distances with P0118, but fix it within a week or two. The engine may run rich, use more fuel and idle rough. Because the temperature gauge may not work, keep an eye out for steam or a sweet smell, and stop if either appears. ### What does ECT sensor circuit high mean? Circuit high means the coolant temperature sensor's signal voltage is near the top of its range. Because voltage rises as coolant gets colder, the computer reads an impossibly cold engine, often -40 degrees. That nearly always means an open circuit, such as a broken wire, an unplugged connector or a failed sensor. ### What causes P0118? A failed coolant temperature sensor is the most common cause of P0118. A loose, unplugged or corroded connector and a broken signal or ground wire are also common. Less often, the signal wire is shorted to another powered circuit. Check the connector first, since it is the easiest and cheapest fix. ### How much does it cost to fix P0118? Most P0118 repairs cost $50 to $400 at an independent shop. Reconnecting or replacing a damaged connector runs $50 to $200, a new coolant temperature sensor $100 to $350, and tracing and repairing a broken wire $100 to $400. Engines with the sensor under the intake cost more. ### Why is my temperature gauge reading cold with P0118? On many vehicles the dashboard gauge uses the same coolant temperature sensor, so an open circuit makes the gauge read cold or stay at the bottom. Other vehicles use a separate sender, so the gauge may still work. Either way, do not rely on the gauge until P0118 is fixed. ### Can P0118 cause poor gas mileage? Yes, P0118 often lowers gas mileage noticeably. The computer thinks the engine is cold, or uses a default value, so it may add extra fuel as if the engine were still warming up. Some cars also run rich at idle and smell of fuel. Mileage usually returns once the circuit is fixed. ### Can I fix P0118 myself? Often, yes. Reseating or cleaning the connector costs nothing, and replacing the sensor is a moderate job on many engines. Work only on a cold engine, have matching coolant ready to top up, and bleed air from the system afterward. Tracing a hidden broken wire may be easier for a shop. ### Why are the cooling fans always on with P0118? Many engine computers turn the cooling fans on full time when the coolant temperature signal is invalid. Since the computer cannot tell how hot the engine really is, it plays safe. The fans should cycle normally again once the open circuit is repaired and the code is cleared. ### Can a bad thermostat cause P0118? No, a bad thermostat does not cause P0118. A thermostat stuck open keeps the engine cool and usually sets P0128, but the sensor still reports a real temperature. P0118 is an electrical fault showing a reading colder than any real engine. Look at the sensor, the connector and the wiring instead. ### Can cold weather cause P0118? Cold weather alone should not cause P0118, because the code threshold sits below any temperature a real engine would see. Cold can, however, shrink a cracked connector or brittle wire enough to open the circuit. If P0118 shows up only on frosty mornings, inspect the connector and harness closely. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) - [Car Care Council: Car care guide](https://www.carcare.org/car-care-guide/) ## Other cooling codes - [P0107 Manifold Absolute Pressure/Barometric Pressure Circuit Low](https://diagxcar.com/codes/p0107) - [P0108 Manifold Absolute Pressure/Barometric Pressure Circuit High](https://diagxcar.com/codes/p0108) - [P0112 Intake Air Temperature Sensor 1 Circuit Low (Bank 1)](https://diagxcar.com/codes/p0112) - [P0113 Intake Air Temperature Sensor 1 Circuit High (Bank 1)](https://diagxcar.com/codes/p0113) - [P0117 Engine Coolant Temperature Sensor 1 Circuit Low](https://diagxcar.com/codes/p0117) - [P0121 Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0121) - [P0122 Throttle/Pedal Position Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0122) - [P0123 Throttle/Pedal Position Sensor/Switch "A" Circuit High](https://diagxcar.com/codes/p0123) - [P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control](https://diagxcar.com/codes/p0125) - [P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)](https://diagxcar.com/codes/p0128) ## Symptoms - [Why is black smoke coming from my exhaust?](https://diagxcar.com/symptoms/black-smoke-from-exhaust) - [Why is my car getting bad gas mileage?](https://diagxcar.com/symptoms/poor-gas-mileage) - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) ## Repair costs - [Thermostat replacement cost](https://diagxcar.com/cost/thermostat-replacement): $150-$450 - [Coolant flush cost](https://diagxcar.com/cost/coolant-flush): $110-$260 --- Source page: https://diagxcar.com/codes/p0118 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.